Protests erupt in US after police shot an unarmed suspect in his own grandmother's backyard.

‘Black Lives Matter‘ protests are erupting across California after an autopsy showed police officers shot an unarmed suspect eight times in the back in the backyard of this grandmother’s home.

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Two officers were called on March 18 to investigate a report that someone was breaking into cars in the South Sacramento neighbourhood of Meadowview. A helicopter joined the search, at one point telling the officers the suspect had been seen picking up a crowbar.

Police chased Stephon Clark, 22, into his own grandmother’s backyard. Video footage from the police car shows an officer yelling “gun” repeatedly and then firing his weapon at Clark almost immediately, The New York Times reports.

Clark was shot eight times and died at the scene. He was not holding a crowbar or a gun. The only thing he was carrying was his cell phone.

Clark’s family, as well as thousands of protesters, belive the 22-year-old’s death is the latest in a long history of police brutality towards African Americans.

The autopsy was conducted by Dr. Bennet Omalu and commissioned by Clark's family. The results, presented on Friday, showed Clark was shot three times in the lower back, twice near his right shoulder, once in his neck, once under his armpit and once in his leg.

Even though the bullet to the leg entered from the front of the thigh, Dr Omalu believes the impact occurred while Clark was falling.

"He was shot from the back," Dr Omalu said Friday at a news conference, adding seven of the shots were potentially fatal and that Clark "bled massively".

A total of 20 shots were fired and police didn't approach Clark until minutes after the shooting stopped.

"We need to know you're okay," the video of the incident shows an officer yelling three minutes after the shooting ended, The New York Times reports.

"We need to get you medics but we can’t go over to get you help unless we know you don’t have a weapon."

The autopsy revealed Clark lived for three to 10 minutes after the shooting, with medical assistance finally arriving six minutes after the bullets were fired.

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Police issued a statement the day after the shooting that they "saw the suspect facing them, advance forward with his arms extended, and holding an object in his hands".

Police video shows Clark had initially moved toward the officers, who are peeking out from behind a corner of the house, but it's not clear he's facing them or that he knows they are there when they open fire after shouting "gun, gun, gun."

"His back was turned - he didn't get a chance," said Latarria McCain, who joined several hundred in a downtown protest that lasted more than four hours on Saturday, AAP reports.

"When a young man who is 22 is shot down in his grandma's backyard, which is supposed to be a safe place, I don't know. What's beyond a crisis?" another protester, Nikki Whitfield, who works at a local adoption agency and attended the community forum, said.

Calls for justice and charges against two police officers aren't abating and the demonstrations saw several hundred members of the black community discussing police brutality and calling out of the names of black people who have been killed by law enforcement in recent years.

Sacramento police responded with a brief statement that said the department had not yet received an official autopsy report from the Sacramento County coroner's office. It said the coroner's death investigation is independent from the investigation being conducted by police and the state Department of Justice.