President Barack Obama has called for calm after a Florida jury found George Zimmerman not guilty of the fatal shooting of unarmed black teenager Trayvon Martin.
"We are a nation of laws and a jury has spoken," he said in a statement. "I now ask every American to respect the call for calm reflection from two parents who lost their young son."
There have been demonstrations in San Francisco, Los Angeles, Philadelphia, New York and Washington DC. In Oakland, California, protesters blocked traffic and smashed the window of a police car.
A jury of six women took 16 hours over two days to give their verdict, which was condemned by some civil rights groups.
Mr Zimmerman appeared stone-faced as the verdict was announced, but showed a slight smile of relief. His parents embraced each other and his wife was tearful.
Mr Zimmerman, 29, said the 17-year-old attacked him on the night of 26 February 2012 in the central Florida town of Sanford.
Prosecutors contended the neighbourhood watch coordinator in his gated community was a "wannabe cop" who tracked down and shot the teenager without justification.
His lawyer, however, said his prosecution was "disgraceful".
The jury could have convicted Mr Zimmerman of second-degree murder or manslaughter.
"Today, justice failed Trayvon Martin and his family," Roslyn M Brock, chairman of the National Association of Colored People, said in a statement.
"We call immediately for the Justice Department to conduct an investigation into the civil rights violations committed against Trayvon Martin. This case has re-energised the movement to end racial profiling in the United States."
The news also drew angry shouts from some of the dozens of demonstrators who had gathered outside the courtroom during the day in support of the teenager’s family. His parents were not in the court during the reading of the verdict.
Civil rights leader Jesse Jackson tweeted within minutes of the acquittal: "Avoid violence, it will lead to more tragedies. Find a way for self construction not deconstruction in this time of despair."
What happened in Sanford that February night may never have gone beyond the back pages of a local newspaper if police had immediately arrested Mr Zimmerman.
But he walked free for more than six weeks after the shooting, because police believed his claim of self-defence, triggering protests and cries of injustice across the US.