NEW YORK, JULY 14: People hold photos of Trayvon Martin at a rally honoring Martin at Union Square in Manhattan on July 14, 2013 in New York City. George Zimmerman was acquitted of all charges in the shooting death of Martin July 13 and many protesters questioned the verdict.
President Barack Obama appealed to Americans for restraint Sunday amid anger from civil rights activists and public protests against the acquittal of a man who gunned down an unarmed black teenager.
A Florida jury late Saturday found neighbourhood watch volunteer George Zimmerman not guilty of murdering Trayvon Martin, after a long and racially-charged trial that transfixed much of the United States for weeks.
Zimmerman, 29, was charged with second degree murder, having pursued Martin, 17, through a gated community in the town of Sanford, eventually shooting him during an altercation on the rainy night of February 26, 2012.
The trial aroused strong passions and divided those who believed that Zimmerman -- whose father is white and whose mother is Peruvian -- had racially-profiled Martin, and those who believed he acted in self-defense.
Spontaneous protests broke out in San Francisco, Philadelphia, Chicago, Washington and Atlanta overnight, following the verdict, though they were mostly peaceful.
On Sunday, a large demonstration in New York attracted several thousand people, with placards that read, "Jail racist killers, not black youth," and "We are all Trayvon. The whole damn system is guilty."
One of the marchers in lower Manhattan wore a t-shirt proclaiming: "I'm black. Please don't shoot?"
However, Obama, the first black US president, urged people to accept the trial verdict.
"We are a nation of laws, and a jury has spoken," Obama said in a statement. "I now ask every American to respect the call for calm reflection from two parents who lost their young son."
Earlier in Oakland, California, protesters had smashed windows and spray painted cars after the verdict was announced live on television.
Obama had spoken somberly on Martin before, noting that if he had a son he would "look like Trayvon."
On Sunday, the president tied the killing of the teenager to the problems surrounding gun use in the United States -- an issue in which he tried but failed to push through new control measures in the US Congress earlier this year.
"We should ask ourselves, as individuals and as a society, how we can prevent future tragedies like this. As citizens, that's a job for all of us. That's the way to honor Trayvon Martin," Obama said.
Florida police initially declined to press charges against Zimmerman, sparking mass protests. He was eventually arrested in April 2012 and charged with second-degree murder.
"Obviously, we are ecstatic. George Zimmerman was never guilty of anything except protecting himself in self-defense," said his lead attorney Mark O'Mara after the verdict.
Memories of the deadly April 1992 riots in Los Angeles, which broke out after a similarly racially-tinged case, still linger among US law enforcement officials.
Fearing violence after the Zimmerman verdict, police were out in force in Sanford, and the crowd of several hundred outside the courthouse was loud at times, but not violent.A racial divide, however, was evident in Sanford pastor Valerie Houston's sermon on Sunday