The parents of an unarmed black teenager who was shot dead by a neighbourhood watch captain in Florida held a rally today in the city where he was killed to protest the lack of an arrest.

Rallies have been held across the U.S. over the shooting of 17-year-old Trayvon Martin a month ago, and U.S. President Barack Obama mentioned the case last week.

George Zimmerman reported Martin to police as suspicious and followed him, despite advice from an emergency dispatcher not to do so. Zimmerman told police he shot in self-defence, and he hasn't been arrested or charged.

Professional athletes joined civil rights leaders Jesse Jackson and Al Sharpton at Monday's rally before a city commission meeting. It's the first time Sanford commissioners have met since the city's police chief stepped down in what he called a temporary move.

The rallies mark the one-month anniversary of the teen's death.

Martin's parents, Tracy Martin and Sybrina Fulton, addressed the commission.

"We are asking for justice," said the teenager's father.

Lawyer denies racial motive

Zimmerman has not spoken in public about the shooting. His lawyer, Craig Sonner, has denied there was any racial motive. Zimmerman's father is white and his mother is Hispanic.

A friend of Zimmerman said Monday that Zimmerman would tell the teen's parents he's "very, very sorry" if he could.

'The only comment that I have right now is that they killed my son and now they're trying to kill his reputation.'—Sybrina Fulton, mother of Trayvon Martin

Speaking on ABC's Good Morning America, Joe Oliver said Zimmerman is not a racist and has virtually lost his own life since the shooting.

"This is a guy who thought he was doing the right thing at the time, and it's turned out horribly wrong," Oliver said.

The state Department of Juvenile Justice has confirmed that Martin does not have a juvenile offender record.

The Orlando Sentinel reported Monday that Zimmerman told police he lost Martin in the neighbourhood and was walking back to his vehicle when the youth approached him from behind. The two exchanged words, Zimmerman said, and Martin then punched him in the nose, jumped on top of him and began banging his head on a sidewalk. Zimmerman said he began crying for help; Martin's family thinks it was their son who was crying out.

The Sanford police statement said the newspaper story was "consistent" with evidence turned over to prosecutors.

School suspension

Fulton also said Monday authorities are trying to demonize her son by leaking information to the news media that marijuana was the reason for his suspension from school.

George Zimmerman, a Sandford, Fla., neighbourhood watch captain, insists he shot unarmed teenager Trayvon Martin in self-defence. (Orange County Jail via Miami Herald/Associated Press)

Sybrina Fulton said Monday that unnamed investigators were trying to destroy Martin's reputation. Sanford police say it's possible the information was leaked to the media, but it was not authorized.

Martin was suspended by Miami-Dade County schools officials after marijuana residue was found in a baggie in his book bag.

"The only comment that I have right now is that they killed my son and now they're trying to kill his reputation," Fulton told reporters.

Martin family attorney Benjamin Crump said the link between the youth and marijuana should have no bearing on the probe into his shooting death. State and federal agencies are investigating, with a grand jury set to convene April 10.

"If he and his friends experimented with marijuana, that is completely irrelevant," Crump said. "What does it have to do with killing their son?"

The Sanford Police Department insisted there was no authorized release of the suspension information but acknowledged there may have been a leak within the agency. City manager Norton Bonaparte Jr. said the source of the leak would be investigated and the person responsible could be fired.

"We do not condone these unauthorized leaks of information," Bonaparte said.