Michael Brown
Michael Brown's family say the 18 year-old was unarmed when he was shot by police in St. Louis, Missouri. Twitter

Police in the US are facing a furious backlash after an unarmed young black teen was shot by police officers.

The St Louis Post-Dispatch reports that police officials have announced that Brown was shot when he was running away from police officers, after struggling for an officer's gun while inside a patrol car. One shot was fired inside the vehicle, and officers fired multiple times as Brown fled.

Eighteen year-old Michael Brown was shot by police officers as he walked to his grandmother's home in Ferguson, an area in the suburbs of St. Louis, Missouri.

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Brown's grandmother, Desiree Harris, told the Associated Press that she was driving on Saturday afternoon when she saw her grandson running close to her home.

"He was running this way. When I got up there, my grandson was lying on the pavement. I asked the police what happened. They didn't tell me nothing."

Harris said her son had recently graduated from high school as was looking forward to starting college classes on Monday.

"My grandson never even got into a fight," she said. "He was just looking forward to getting on with his life. He was on his way."

His mother, Lesley McSpadden, said of the police: "You're not God, you don't get to decide when you get to take somebody from here. I know they killed my son. This was wrong and it was cold-hearted.

"[My son] doesn't kill, steal or rob. He doesn't do any of that", she added.

In the hours after the shooting tempers flared as outraged local residents gathered where the shooting took place to protest chanting "We Are Michael Brown" and "Kill the police".

About 60 police units, including armed units and officers with police dogs, were used to calm tensions on Saturday evening. Outside the Ferguson Police headquarters about 100 protestors gathered chanting "No justice, No peace!"

Brown's stepfather Louis Head was pictured on social media holding a sign that read: "Ferguson Police just executed my unarmed son!!!"

Ferguson Police have not released any information about the shooting. Ferguson Police Chief Thomas Jackson said his department immediately turned the matter over to the St. Louis County police.

St. Louis County Police Chief Jon Belmar confirmed the county police will investigate the shooting, rather than the local force, with the possible assistance of the FBI.

"We are hoping for calm and for people to give us a chance to conduct a thorough investigation," he said.

County police spokesman Brian Schellman promised a lengthy investigation. "We're still trying to piece together what happened and why," he added.

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This is the latest in a series of recent incidents when unarmed young black men have been shot and killed by police officers. Earlier this month a medical examiner ruled New Yorker Eric Garner was killed by an NYPD officer, Daniel Pantaleo, who held him in an illegal chokehold during an arrest for selling individual cigarettes.

In 2012 17-year-old Trayvon Martin was shot and killed in a racially charged shooting by George Zimmerman, a Florida neighbourhood watch organiser who was subsequently acquitted of murder charges.