(Update with quotes from prayer vigil, paragraphs 9-12)
By Renita Young
CHICAGO, Sept 20 (Reuters) - Chicago police on Friday searched for suspected gang members who opened fire with an assault weapon at a park, wounding 13 people including a 3-year-old boy, in an attack that has highlighted Mayor Rahm Emanuel's struggle to stem violence.
Up to three gunmen used at least one assault-style weapon with a high-capacity ammunition clip to fire on a crowd in the city's Back of the Yards neighborhood late on Thursday, Chicago Police Superintendent Garry McCarthy said.
The shooting, on a warm night as residents were enjoying a basketball game, came 10 days after Emanuel and McCarthy held a press conference nearby to claim success in a strategy of flooding 20 high-crime neighborhoods with police.
Back of the Yards was not among the neighborhoods receiving police reinforcements under the plan, McCarthy said.
He blamed lax U.S. gun control laws for allowing assault weapons on the streets and complicating efforts to curb violence in Chicago, which has one of the worst murder rates in the country and sees more violent crime than the more populous cities of New York and Los Angeles.
"A military-grade weapon on the streets of Chicago is simply unacceptable," McCarthy said. "This kind of shocks the consciousness just like the other high-profile incidents that happen across this country.
The Chicago shooting followed the killing of 12 people at the Navy Yard in Washington on Monday by a man with a history of gun-related offenses and treatment for mental illness.
Emanuel's office said he canceled meetings in Washington on Friday and returned to Chicago, where he went to a hospital to visit Deonta Howard, the 3-year-old boy police said was critically wounded by a bullet entering his ear.
Late on Friday the mayor attended a prayer vigil during which he urged witnesses to come forward and the shooters to surrender.
"For a city to have its sense of civility, its sense of community, it must live by a moral code, not a code of silence," Emanuel said at the New Beginnings Church, some 5 miles (8 km) west of where the shooting occurred.
"So many times in our community when violent things happen, they think we're savages, they think we're animals, they think that we're anything but human," New Beginnings Pastor Corey Brooks said at the vigil, which drew about 60 people.
"When our kids get shot, it hurts us, it pains us. When our community has to endure violence it hurts us and it pains us as well."
'JUST STARTED SHOOTING'
Emanuel, a former White House Chief of Staff for President Barack Obama, promised when he was elected mayor in 2011 to make the streets of Chicago safe for everyone.
According to witnesses, gunfire rang out as a crowd was watching basketball.
"We were sitting in the park talking, then two dudes came up, stood by the gate for a minute, then they just started shooting," said Michelle Adams, 16.
Other witnesses said the gunmen had driven up and fired on the basketball court from their vehicle.
Police said that, aside from the toddler, the injured ranged in age from 15 to 41.
"It's a miracle there were no fatalities," McCarthy said.
Three of the gunshot victims including Howard were in serious to critical condition in area hospitals. McCarthy said none of the injuries were life threatening.
Naphtali Dukes, 38, who identified himself as Howard's cousin, said he had been told the boy would survive.
Mitchell Gary, 53, who was nearby when the shooting occurred, said his brother was shot in the back and buttocks and his nephew was shot in the arm and ankle.
"I was walking up the street and all of a sudden I heard gunshots," Gary said. "When I got there, everybody was screaming, then I see my brother and nephew laying on the ground shot."
There were more than 500 murders in Chicago in 2012, according to a report this week by the FBI. New York City, with three times Chicago's population, had 419 murders in 2012, the FBI said.
One Chicago murder this year which caught national attention was of Hadiya Pendleton, 15, an honor student killed at a park just days after she performed at a January presidential inauguration event in Washington.
Obama, who considers Chicago his hometown, tried to tighten the nation's gun control laws after 26 people, among them 20 children, died in a shooting rampage last December in Newtown, Connecticut.
The effort failed in Congress after the powerful gun rights lobby, the National Rifle Association, opposed tougher laws.
Chicago and Illinois have loosened some gun laws this year despite the opposition of Emanuel.
The Chicago City Council abolished its registry of gun owners after its gun control law was ruled unconstitutional by the courts. This summer Illinois adopted a law allowing residents to carry concealed guns.
Crime is one of a host of issues confronting Emanuel. Citing budget problems, the city closed 50 public schools in May, angering parents and the teachers union.
The city also faces large and growing pension liabilities, and Emanuel's former senior finance official was recently indicted in Ohio, sparking an investigation of the official's dealings in Chicago. (Additional reporting by Tom Brown, Sharon Bernstein, Edith Honan and David Bailey; Writing by Nick Carey; Editing by Greg McCune, Krista Hughes and Xavier Briand)