NEW YORK -- Thousands of New Yorkers donned "I Run for Boston" bibs during a 4-mile run Sunday in Central Park, one of a number of races held around the world in support of the victims of the marathon bombings.

"It was really quite a powerful morning," said Mary Wittenberg, CEO of the New York Road Runners.

Wittenberg said later Sunday at another run dedicated to victims that she had been in close communication with Boston Marathon organizers.

"This is one community," Wittenberg said. "After 9/11, we were all New Yorkers. After last Monday, we're all Bostonians. And I just want to add, I think now we've got one world of runners."

More than 6,000 runners took part in Sunday's City Parks Foundation Run for the Parks, which was planned before Monday's bombings.

Organizers turned it into a show of solidarity by selling "I Run for Boston" T-shirts with proceeds going to the One Fund Boston, the official fund for those affected by the bombing.

Other "Run for Boston" events have taken place around the U.S. and the world, with many runners wearing blue and yellow, the official Boston Marathon colors.

More than 500 runners gathered in St. Louis on Saturday for a Unity Run. In San Francisco, about 400 people ran 4 miles along the Embarcadero on Friday. A run christened "Boston Strong San Diego" is planned for Monday.

In Michigan, runners braved sub-freezing temperatures and a partly flooded course Sunday for the Lansing Marathon, which was dedicated to the Boston victims. In Burlington, Vt., a 5-kilometer walk-run on Saturday raised more than $10,000 for Massachusetts General Hospital's emergency fund and One Fund Boston.

In Greece, marathon runners wore black bracelets with the text "Thessaloniki to Boston" and several also wore a label on their running gear that read "Why?" There was a visible presence of extra security at the "Alexander the Great" marathon compared to previous races.

Sunday's London Marathon started with tributes to the Boston victims and a moment of silence. The London event was the first major race since the twin bombings in Boston killed three people and injured more than 180 others.

In New York, security for Sunday morning's run was tightened in response to the attacks.

Runners were told not to bring bags. Instead, clear plastic bags were provided by race officials for those who brought belongings.

The afternoon run, also in Central Park, was organized through an email chain among running groups and drew several hundred runners.

Among them was Elle Green, a New Yorker wearing a bright yellow shirt from Monday's Boston Marathon.

Green said she had just finished the marathon when she heard the first explosion.

"I saw a couple of people loaded into ambulances," she said. "It was pretty horrific."

Green said runners are "a special breed" who take care of one another on and off the course.

"It's such an innocent activity," she said. "It's just such a positive experience, and for something like that to happen, it tears at our hearts."

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  • Boston Marathon Explosion

    Police officers with their guns drawn hear the second explosion down the street. The first explosion knocked down a runner at the finish line of the 117th Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Newspapers are on sale at a stand on Newbury Street on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others. Darren McCollester / Getty Images)

  • WARNING

    Some photos in this slideshow are graphic and may be disturbing to some viewers.

  • Boston Marathon Explosion

    In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (Kenshin Okubo / The Daily Free Press / AP)

  • Boston Marathon Explosion

  • Boston Marathon Explosion

    A woman kneels and prays at the scene of the first explosion on Boylston Street near the finish line of the 117th Boston Marathon on April 15, 2013. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Victims are in shock and being treated at the scene of the first explosion that went off near the finish line of the Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    People tend to an injured woman on the corner of Exeter and Newbury Streets after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Bill Greene / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Bystanders help an injured woman at the scene of the first explosion on Boylston Street near the finish line of the 117th Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Medical workers runs an injured man past the finish line the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (Charles Krupa / AP)

  • Boston Marathon Explosion

    Police and federal officials exit an apartment complex at 364 Ocean Avenue with a possible connection to the earlier expolsions during the Boston Marathon on April 15, 2013 in Revere, Massachusetts. Three people are confirmed dead and at least 141 injured after two explosions went off near the finish line to the marathon. (Darren McCollester / Getty Images)

  • Boston Marathon Explosion

    A victim of the first explosion is helped on the sidewalk of Boylston Street, after two explosions went off near the finish line of the 117th Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Unclaimed finish line bags are viewed near the scene of a twin bombing at the Boston Marathon, on April 16, 2013 in Boston, Massachusetts. Three people are confirmed dead and at least 141 injured after the explosions went off near the finish line of the marathon yesterday. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Spencer Platt / Getty Images)

  • Boston Marathon Explosion

    A heavily armed Boston police officer (R) and a National Guard soldier (L) stands guard in front of the Taj Hotel April 16, 2013 in Boston, Massachusetts, in the aftermath of two explosions that struck near the finish line of the Boston Marathon April 15. A massive probe was underway Tuesday after two bombs struck the Boston Marathon, killing at least three and wounding more than 100. Monday's blasts near the finishing line raised fears of a terrorist attack more than a decade after nearly 3,000 people were killed in suicide airliner strikes on New York, Washington and Pennsylvania on September 11, 2001. US President Barack Obama went on national television to warn against "jumping to conclusions" but a senior White House official, speaking on condition of anonymity, said such an attack was "clearly an act of terror." (Stan Honda / AFP / Getty Images)

  • Boston Marathon Explosion

    A man comforts a victim on the sidewalk at the scene of the first explosion near the finish line of the 117th Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images

  • Boston Marathon Explosion

    Emergency personnel respond to the scene after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (David L. Ryan / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Boston Police look at blown out windows at the scene of the first explosion on Boylston Street near the finish line of the Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: Police officers with their guns drawn hear the second explosion down the street. The first explosion knocked down a runner at the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

  • Boston Marathon Explosion

    Injured people lie on the sidewalk near a barrier at the scene of the first explosion that went off near the finish line of the 117th Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: Emergency personnel respond to the scene after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    Passersby put pressure on a victim's leg to try to stop the bleeding at the scene of the first explosion near the finish line of the Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    The marathon finish line bridge is seen on Boylston Street on April 16, 2013 in Boston, Massachusetts. on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others. (Darren McCollester / Getty Images)

  • Boston Marathon Explosion

    Blood from victims covers the sidewalk on Boylston Street, at the site of an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. At the right foreground is a folding chair with the design of an American flag on the cover. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (Charles Krupa / AP)

  • Boston Marathon Explosion

    A runner reacts near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Alex Trautwig / Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene in a wheelchair. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    A runner passes a police officer dressed in tactical gear, who blocks a road leading to the Boston Marathon route, the morning after explosions killed three and injured more than 140 in Boston, Tuesday, April 16, 2013. The bombs that blew up seconds apart at the finish line of one of the world's most storied races left the streets spattered with blood and glass, and gaping questions of who chose to attack at the Boston Marathon and why. (Charles Krupa / AP)

  • Boston Marathon Explosion

    A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene in a wheelchair. (David L. Ryan / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    Carlos Arredondo, who was at the finish line of the 117th Boston Marathon when two explosives detonated, leaves the scene on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 28 injured after at least two explosions went off near the finish line to the marathon. (Darren McCollester / Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene on a stretcher. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    Officials react as the first explosion goes off on Boylston Street near the finish line of the 117th Boston Marathon on April 15, 2013. (John Tlumacki / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    First responders tend to the wounded after two explosions occurred along the final stretch of the Boston Marathon on Boylston Street in Boston, Massachusetts, U.S., on Monday, April 15, 2013. (Kelvin Ma / Bloomberg / Getty Images)

  • Boston Marathon Explosion

    Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: Two officials run away from the first explosion, right, on Boylston Street at the 177th Boston Marathon, April 15, 2013. (Photo by John Tlumacki/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    A person who was injured in the first explosion is wheeled across the finish line of the Boston Marathon. (John Tlumacki / The Boston Globe / Getty Images)

  • Explosions At 117th Boston Marathon

    BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene on a stretcher. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

  • Boston Marathon Explosion

    Neighbors hug outside the home of the Richard family in the Dorchester neighborhood of Boston, Tuesday, April 16, 2013. Martin Richard, 8, was killed in Mondays bombing at the finish line of the Boston Marathon. (Michael Dwyer / AP)

  • Boston Marathon Explosion

    First responders rush to help injured people after two explosions occurred along the final stretch of the Boston Marathon on Boylston Street in Boston, Massachusetts, U.S., on Monday, April 15, 2013. Two powerful explosions rocked the finish line area of the Boston Marathon near Copley Square and police said many people were injured. (Kelvin Ma / Bloomberg / Getty Images)

  • Boston Marathon Explosion

    Emergency workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

  • Boston Marathon Explosion

    There was smoke and panic in the street as emergency personnel responded to the scene after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (David L. Ryan / The Boston Globe / Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene on a stretcher. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: Emergency personnel respond to the scene after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: A man lays on the ground after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    BOSTON - APRIL 15: Two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

  • Boston Marathon Explosion

    Medical workers wheel the injured across the finish line during the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • An unidentified Boston Marathon runner, center, is reunited with loved ones near Copley Square following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

  • Boston Marathon Explosion

    Medical responders run an injured man past the finish line the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

  • Boston Marathon Explosion

    Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

  • Boston Marathon Explosion

    A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)