Police have flooded a Los Angeles suburb where demonstrators have turned violent.
Hundreds of people were taking part in a march to protest a jury's acquittal of George Zimmerman, the neighbourhood watch patrolman who shot dead an unarmed black teenager in Florida.
There is growing anger over the jury's verdict.
US President Barack Obama has appealed for calm, saying the court decision should be respected.
The six-member jury that acquitted Zimmerman did not believe race played a role in the shooting death of Trayvon Martin, the first juror to speak publicly about the trial says.
The juror, identified only as Juror B37, told CNN's Anderson Cooper on Monday night she and the other jurors believed the screaming voice captured on a 911 recording belonged to Zimmerman, and lent little credibility to a key witness who spoke to Martin on the phone before he died. They also put limited faith in the testimony provided by Martin's parents.
"They said anything a mother and father would say," said the juror, who said she wanted to keep her identity secret and was almost invisible in deep shadow.
Juror B37 said she believed Martin was the aggressor - and added she had "no doubt" that Zimmerman feared for his life during the confrontation in a Sanford, Florida, housing complex.
"I think his heart was in the right place," she said of Zimmerman. "It just went terribly wrong."
"Do you think he's guilty of something?" Cooper pressed.
"I think he's guilty of not using good judgment," she responded.
Zimmerman, the juror said, was "overeager to help people". She added that she believed he was "was pretty consistent and told the truth" to investigators.
Although Juror B37 said she was sympathetic to Zimmerman throughout the trial, she said the jury was initially divided. Three members of the all-female jury wanted to find Zimmerman not guilty, two believed a manslaughter verdict was appropriate, and one supported a finding of second-degree murder.
All six members found the law to be "very confusing", she said.
But after reading the definitions of second-degree murder and manslaughter "over and over", the jurors reached consensus.
"He had a right to defend himself," Juror B37 said of Zimmerman. "If he felt threatened, that his life was going to be taken away from him or he was going to have bodily harm, he had a right."
Before ending her interview with Cooper, Juror B37 broke down in tears.
"I want people to know that we put everything into everything to get this verdict," she said. "We didn't just go in there and say, we're going to do guilty/not guilty. We thought about it for hours and cried over it afterwards. I don't think any of us could ever do anything like that ever again."
What is known about Juror B37 came from her comments as she was being interviewed by both sides before the trial began, that she is middle-aged, has two grown children, once had a license to carry a concealed weapon and worked for a chiropractor for 16 years.
Despite keeping her identity secret, Juror B37 has already secured an agent to peddle an as-yet unwritten book.