Grainy mobile phone footage showing a police officer shooting an unarmed man in the back as he lay face down on the ground have been played repeatedly on YouTube, prompting street riots in the United States.

The New Year's Day death of 22-year-old Oscar Grant in Oakland, California, led to violent protests in the area after several videos of the incident on a train station platform were uploaded onto the website. One clearly shows the shooting.

The officer, who has resigned from the force, has not been charged with any wrongdoing. Some experts have questioned whether he fired his gun deliberately or mistakenly believed he was using his stun gun instead.

The shooting unfolded in front of dozens of train passengers who were returning home after New Year's Eve celebrations. Police arrived shortly after midnight on New Year's Day at the Fruitvale Bay Area Rapid Transit station following reports of young men fighting on a train. Mr Grant was one of several who were ordered off the train, questioned and then restrained by transit police officer Johannes Mehserle, 27, and other officers.

Videos shot by onlookers on camera phones show Mr Grant being pushed onto his stomach shortly before Mehserle fired his gun at his back. The bullet ricocheted off a pavement and pierced his lung, killing him.

The victim's family claims that the officer shot him deliberately but some have argued that he may have thought he was firing his stun gun.

In 2006, a sheriff's deputy in Washington state shot and wounded a disturbed man after mistakenly using his .40-caliber gun instead of his stun weapon. Outrage over the shooting has been fueled by raw video clips posted on YouTube and various news web sites. Over the past week, video of the shooting has been viewed more than 500,000 times on the web site of KTVU-TV.

A group of angry demonstrators smashed store windows late on Wednesday, set fire to cars and clashed with officers equipped with riot gear and tear gas in downtown Oakland. More than 100 people were arrested and about 300 businesses were damaged.

Oakland Mayor Ron Dellums called for calm. "Even with our anger and our pain, let's still address each other with a degree of civility and calmness and not make this tragedy an excuse to engage in violence," he said. "I don't want anybody hurt. I don't want anybody killed."

Extra police were posted at Bay Area Rapid Transit stations after the rioting and a demonstration passed off relatively peacefully yesterday. At the mayor's request, the Oakland Police Department has launched an investigation into the shooting.

Mehserle was initially placed on paid leave. He resigned from the BART police force on Wednesday, but officials say he has refused to speak with the transit agency's investigators.

Mr Grant's family has filed a $25 million wrongful-death claim against BART, the San Francisco Bay Area's commuter rail system, and relatives want Mehserle to be criminally charged.

"They want justice, but they don't want any more violence," said John Burris, an attorney for Grant's family. "That officer hasn't been prosecuted ... That's why people don't have confidence in the system right now."

Local African-American leaders expressed outrage at the shooting and some Oakland residents have alleged it was racially motivated.

BART officials said the agency is trying to conduct a thorough investigation, but that the public appears to be making judgments about the case based on raw video online or on television.

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