Angry protesters marched in US cities after George Zimmerman, a neighbourhood watch volunteer, was found not guilty by a Florida jury in the shooting death last year of Trayvon Martin, an unarmed black teenager.
Spontaneous marches erupted in cities including San Francisco, Chicago, Washington, Atlanta and Philadelphia, with reports of sporadic acts of violence.
In a case that drew national attention for its implications on race and guns, yesterday's decision was made by six female jurors, all but one of them white. They spent two days weighing whether the shooting of Martin was a crime or self-defence.
After the verdict was announced, Seminole County Circuit Judge Debra Nelson dismissed the jury and told Zimmerman - whose father is white and whose mother is Peruvian - that he was free to go.
Zimmerman, 29, had been accused of pursuing Martin, 17, through a gated community in Sanford, Florida, and shooting him during an altercation on a rainy night in February last year.
The killing led to mass marches in several US cities after police initially declined to press charges against Zimmerman.
Martin's family maintained the teen was not the aggressor, and prosecutors suggested Martin was scared because he was being followed by a stranger. Defence lawyers, however, claimed Martin knocked Zimmerman down and was slamming the older man's head against a footpath when Zimmerman fired his gun.
According to Florida's controversial "Stand Your Ground" law, people who fear for their lives can use deadly force to defend themselves without having to flee a confrontation.
"Obviously, we are ecstatic with the results. George Zimmerman was never guilty of anything except protecting himself in self-defence," said his lead lawyer Mark O'Mara.
Martin's mother and father were not in the courtroom when the verdict was read; supporters of his family who had gathered outside yelled "No! No!" on hearing the news.
Andrew Perkins, 55, a black resident, angrily asked outside the courthouse: "How the hell did they find him not guilty? He killed somebody and got away with murder," Perkins shouted.
Agence France-Presse, Reuters, Associated Press