A Florida police chief criticised over the investigation into the shooting of an unarmed black teenager has announced he will temporarily step down.

Bill Lee has been censured by officials in Sanford, an Orlando suburb, over the death of 17-year-old Trayvon Martin.

Florida Governor Rick Scott, meanwhile, announced he had appointed a new prosecutor to lead the investigation.

Neighbourhood watchman George Zimmerman, 28, was not charged for shooting the teenager.

He said he was defending himself because Mr Martin had attacked him. Mr Martin was on his way home after buying sweets from a local shop, his parents say.

In Florida, a law known as "stand your ground" can prevent criminal or civil prosecution when deadly force is used in self-defence.

'City in turmoil'

Mr Lee explained his decision at a news conference on Thursday afternoon.

Analysis Thursday's lakeside rally began gradually with a steady trickle of attendees filing into the park carrying lawn chairs and handmade signs in support for Trayvon Martin. It was an idyllic setting for a rally of such seriousness - sailboats bobbed in the nearby water and birds fluttered across palm trees. Soon the crowd began to swell. A man played speeches of Dr Martin Luther King Jr from a speaker attached to his motorcycle. Another man roamed the crowd playing jazz standards on his saxophone. The long list of speakers on stage energised the crowd with various topical rallying cries. "Pick the killer up," yelled one speaker. "Pick him up, pick him up," the crowd chanted back. Meanwhile, Sanford's police department provided security - the very department many citizens believe has been a key part of their town's problems.

"It is apparent that my involvement in this matter is overshadowing the process," he said.

"Therefore I have come to the decision that I must temporarily remove myself."

He added: "I do this in the hope of restoring some semblance of calm to the city, which has been in turmoil for several weeks."

Later on Thursday, Governor Scott announced that state attorney Norman Wolfinger would stand aside from the case.

In a letter to the governor, Mr Wolfinger said his move was aimed at "toning down the rhetoric and preserving the integrity of the investigation".

The governor also appointed a task force led by Lt Gov Jennifer Carroll, an African-American, to conduct hearings on the case and to make recommendations for any changes to state law.

Gov Scott appointed Angela Corey, the state attorney for Jacksonville, to take over the case.

Police chief Lee's decision to stand aside comes a day after city commissioners in Sanford issued a vote of no-confidence in him.

They voted 3-2 to censure the police chief, who has held his position for just 10 months.

Thousands of people attended a mass rally led by civil rights leader Reverend Al Sharpton on Thursday evening in Sanford to demand justice for the 17-year-old.

Tracy Martin, the victim's father, was cheered as he told demonstrators: "The temporary step down of Bill Lee is nothing. We want an arrest, we want a conviction."

Mr Martin's parents have met officials from the Department of Justice, who have launched a civil rights investigation into police conduct of the case.

Nearly one million people have signed online petitions calling for justice.

Mr Sharpton said: "We came for permanent justice. Arrest Zimmerman now! Investigate the entire police department now."

The civil rights leadertweetedearlier that he was attending the rally despite the death of his mother, who passed away on Wednesday.

Meanwhile, Mr Zimmerman has been removed from Seminole State College, where he was studying,the Orlando Sun Sentinel newspaper reports.

"Due to the highly charged and high-profile controversy involving this student, Seminole State has taken the unusual but necessary step this week to withdraw Mr Zimmerman from enrolment," the college said.

On Wednesday, Mr Martin's parents addressed a mass rally in New York to call for the arrest of Mr Zimmerman.

Although Mr Martin was killed in late February, the publication of 911 emergency calls and sworn testimony from a friend have fuelled debate over whether the shooting was truly a case of self-defence.

A Florida grand jury is considering whether there is enough evidence to file charges, and the US justice department has launched a probe into the local police investigation.

Ina statementSanford City Manager Norton Bonaparte called the death of Mr Martin a "tragic situation".

But he emphasised that officers of the Sanford police department were "prohibited from making an arrest based on the facts and circumstances they had at the time".

Transcripts of phone calls released by police indicate that Mr Zimmerman called the police after seeing Mr Martin, describing him as "real suspicious".

The recordings suggest the police dispatcher told Mr Zimmerman not to pursue the teenager.

Mr Zimmerman has said he had shot Mr Martin after the teenager attacked him. Details of the alleged confrontation remain unclear.

Florida's self-defence law, enacted in 2005 and known as "stand your ground", gives people scope to use deadly force rather than retreat during a fight.