* U.S. Justice Department to investigate case
* New York mayor vows to retrain police officers
* Other cities with protests include Washington, Boston, Chicago (Adds details from New York, Washington, Chicago, Boston protests)
By Robert MacMillan and Frank McGurty
NEW YORK, Dec 4 (Reuters) - Mostly peaceful protests flared for a second night on Thursday over a New York grand jury's decision declining to bring criminal charges against a white police officer in the choking death of an unarmed black man.
The reaction in New York and other cities to Wednesday's decision not to indict officer Daniel Pantaleo for his role in the videotaped confrontation that left 43-year-old Eric Garner dead echoed a wave of outrage sparked nine days earlier by a similar outcome in the fatal shooting of an unarmed black teenager by a white policeman in Missouri.
U.S. Attorney General Eric Holder on Thursday promised a full investigation of the New York case.
Pantaleo could still face disciplinary action from an internal police investigation, his lawyer said, adding that he expects that process to move quickly and that his client would be exonerated.
A departmental investigation will likely focus on whether Pantaleo employed a chokehold, banned by New York Police Department regulations, in restraining Garner as he and other officers sought to arrest him for allegedly selling cigarettes illegally on a Staten Island sidewalk in July.
In addition to triggering protests around the country, the New York and Missouri cases have re-ignited debate over a U.S. law enforcement system widely perceived to unfairly target African Americans and other minorities.
SECOND NIGHT OF PROTESTS
Thousands of marchers snaked through the streets of Manhattan for a second night on Thursday, beginning at the evening rush hour and picking up recruits along the way, often weaving between cars and trucks and bringing traffic to a near standstill.
Tensions rose as a crowd of at least 3,000 congregated in Times Square about an hour before midnight, shouting at police, "Who do you protect?" as hundreds more officers moved in steadily to force protesters back onto sidewalks. There were a number of arrests, but no overt outbursts of violence.
Several police helicopters clattered above the skyscrapers overhead.
Hours earlier, protesters parading through lower Manhattan staged sporadic sit-ins at intersections before police in riot gear approached and warned them to move on or face arrest. Most of the marchers complied, and the atmosphere among the racially mixed crowd was boisterous, upbeat and mostly peaceful.
Sharon Gordon, 52, of Matawan, New Jersey, said she hoped politicians would take heed of the public outcry. "There's been a confluence of social media and outrage," she said. "I do believe for the first time we're about to make a change."
A second and third wave of marchers later crossed two bridges into Manhattan from Brooklyn, briefly closing both spans to traffic, then converged on Manhattan's southern tip, at the ferry terminal for Staten Island.
The main group of demonstrators, meanwhile, headed west and briefly closed the West Side Highway along the Hudson River, resulting in at least a handful of arrests, before turning north again through Greenwich Village and Chelsea.
A smaller crowd confronted police along the highway with taunts. Chesray Dolpha, 31, yelled at the officers: "We are not violent. We are not touching you. What are you doing with that baton, brother?" The police made eye contact but did not reply.
Thursday's rallies in New York and elsewhere were generally larger and more numerous on Thursday than the previous night.
Hundreds took to the streets of the nation's capital, chanting, "No justice, no peace, no racist police," as they marched by the U.S. Justice Department, passed near the White House and headed on to the Washington Monument. Protesters staged a "die-in" there, sprawling on the roadway to block traffic.
In Minneapolis, dozens of protesters blocked northbound traffic on Interstate 35W, at times marching or lying down in the middle of the highway, escorted by police in squad cars seeking to keep demonstrators moving.
Protesters in Chicago briefly disrupted traffic on Lake Shore Drive, and a crowd of demonstrators, reported by the Boston Globe to have numbered in the thousands, gathered at the annual holiday tree-lighting ceremony on the Boston Common.
CHOKEHOLDS AND RETRAINING
Unlike the Aug. 9 fatal shooting of 18-year-old Michael Brown by a policeman under disputed circumstances in the St. Louis suburb of Ferguson, Missouri, Garner's encounter with New York City police was captured on video that went viral online.
The clip from a bystander's mobile phone shows Pantaleo grabbing Garner from behind with his arm wrapped around Garner's throat before he was wrestled to the pavement by Pantaleo and three other officers. While being subdued, Garner is heard repeatedly gasping, "I can't breathe" - a phrase that has become a rallying cry by demonstrators.
Pantaleo's lawyer, Stuart London, said in an interview Thursday that his client testified to the New York grand jury that he never put pressure on Garner's neck. Instead, Pantaleo said he used a proper takedown technique, London said.
Patrick Lynch, president of the patrolmen's union, agreed, calling Pantaleo a "model" officer at a news conference.
The city's medical examiner has said police officers killed Garner by compressing his neck and chest, adding that Garner's asthma and obesity had contributed to his death.
Although chokeholds are barred by New York City police regulations, the 2,000-page patrol guide is vague about whether such moves are permitted under certain circumstances, said Maria Haberfeld, who heads the law and criminal justice department at John Jay College.
That gray area, she said, may have influenced the grand jury and could be a factor in the departmental probe.
New York City Mayor Bill de Blasio, who took office in January promising to improve relations between minorities and police, told reporters on Thursday the city's thousands of patrol officers would undergo extensive retraining.
"The relationship between police and community has to change," he told a news conference. "People need to know that black lives and brown lives matter as much as white lives." (Additional reporting by Laila Kearney, Frank McGurty, Sascha Brodsky and David Ingram in New York, Fiona Ortiz and Kim Palmer in Cleveland, David Bailey in Minneapolis and Daniel Wallis in Denver, Julia Edwards in Washington; Writing by Steve Gorman and Joseph Ax; Editing by Scott Malone, Grant McCool, Frances Kerry and Ken Wills)