One of the jurors in the George Zimmerman trial has said the case was not about race.

She said the Florida neighbourhood watch volunteer was justified in firing the shot that killed Trayvon Martin because he feared for his life.

Juror B-37, a mother-of-two who grew up in a military family and used to have a permit to carry a concealed weapon, said she did not believe Mr Zimmerman called police to report a suspicious person because the teenager was black.

The juror, with her identity concealed, told a CNN reporter: "All of us thought race did not play a role." 

She said "he had a right to defend himself" and added that she believed that Trayvon threw the first punch.

"I think Trayvon got mad and attacked him," said the juror, who, according to her literary agent, has abandoned plans to write a book about why she found Mr Zimmerman not guilty of murder.

A jury of six anonymous women found Mr Zimmerman not guilty of second-degree murder and manslaughter last Saturday.

There was a three-week trial in which defence lawyers argued that Mr Zimmerman shot Trayvon in self-defence.

After the verdict, civil rights activists called for federal charges against Mr Zimmerman, saying the trial failed to serve justice.

Police in California made several arrests late yesterday as anger at the acquittal flared in Los Angeles and Oakland.

Los Angeles police arrested one person after a prayer vigil for Trayvon turned rowdy, while in Oakland, police made multiple arrests at a street protest. Officers did not say how many people were arrested.

The juror told CNN that an initial poll of the six women showed three favoured not guilty, two voted for manslaughter and one opted for second-degree murder.

"There was a couple of them in there that wanted to find him guilty of something. And after hours and hours and hours of deliberating over the law, and reading it over and over and over again, we just decided there's no other way or place to go" but acquittal, she said.

Sounding tearful at times, she said the jury worked hard to reach a verdict, and cried over the decision.

"I want people to know that we put everything into it," she said. "We thought about it over and over."

On one crucial point of contention, she said most, if not all, six jurors believed it was Mr Zimmerman and not Trayvon who was calling for help in the background of a 911 emergency call.

Trayvon's mother and brother testified it was Trayvon's voice screaming for help while Mr Zimmerman's parents and several friends swore they recognised it as his voice.

"I think it was George Zimmerman's. All but probably one [juror agreed]," she said.

Where she did fault Mr Zimmerman was for showing bad judgement after calling police to report a suspicious person.

Mr Zimmerman got out of his car after an emergency dispatcher had admonished him not to follow the person.

Juror B-37 said the altercation could have been avoided if both men had just walked away.