LOS ANGELES (AFP, REUTERS) - Unarmed Californian Stephon Clark was shot eight times - mostly in the back - by police in Sacramento, according to a private autopsy released on Friday (March 30) that said he lay dying for several minutes.

The Clark family attorney, celebrated civil rights lawyer Benjamin Crump, said the examination showed no entry wounds in the front of his body, demonstrating that the 22-year-old could not have been a threat to police.

The news has sparked outrage among activists who vowed to take to the streets for a fourth day of protests that have severely disrupted life in the city, although violence and arrests have been minimal.

Hours after the autopsy results, about 200 demonstrators squared off with police in riot gear overnight in California's capital and more protests are planned for Saturday.

Waving signs and chanting the 22-year-old's name in unison, the protesters gathered at city hall before marching into the Old Sacramento part of the city, filled with bars, restaurants and tourists. Protesters, some with megaphones and black masks covering their faces, shouted "Shoot us down, we shut you down", along with expletives directed at the police.

More than 80 police and California Highway Patrol officers in riot gear blocked protesters from marching onto a highway.

Earlier, Crump told reporters Clark was "slain in another senseless police killing under increasingly questionable circumstances" after being chased to his backyard in the California capital by officers who fired 20 rounds.

The review was conducted by high-profile pathologist Bennet Omalu, the former chief medical examiner in San Joaquin County, who is highly regarded for his work on football-related concussion.

Omalu said in a statement he found "four entry wounds in the lower part of Stephon's back; one in the side of his neck, with an exit wound elsewhere in his neck; one in the back of his neck; one under an armpit entering from the side, with an exit wound on the other side of his body; and one in the outside of a leg."

The incident was triggered by an emergency 911 phone call late on March 18 stating that a man was smashing car windows in the neighbourhood.

Clark appeared to fit the suspect's profile and officers chased him, backed by a helicopter equipped with infrared cameras.

Clark - who police say remains the prime suspect - was recorded by the helicopter and police body-cams running through the neighbourhood before entering his backyard.

The officers burst into the yard with weapons drawn and confronted the father-of-two before opening fire.

The officers - one of whom is black - were put on leave but days of disruptive protests followed the incident, which has revived a recurring debate over police abuses against African Americans.

"The proposition that he was facing the officers is inconsistent with the prevailing forensic evidence," Omalu told a news conference in Sacramento.

"He was facing the house, with his left to the officers. He wasn't facing the officers. His left back was facing to the officers."

Omalu said it took three to 10 minutes for Clark to succumb to his injuries, emphasising that "it was not an instant death".

Sacramento's Black Lives Matter chapter announced an evening protest at City Hall as news of the autopsy sparked a fresh wave of anger in the area, where 16 people - three unarmed - have died in confrontations with law enforcement in the last two years.

In a separate shooting case, a white Louisiana police officer was fired on Friday and a second suspended for the killing of Alton Sterling, a black man shot in a 2016 incident that inflamed the US debate on racial bias in law enforcement, a police official said.

Baton Rouge officer Blane Salamoni, who shot Sterling in a confrontation outside a convenience store, was dismissed for violating department standards on use of force and for losing his temper, Police Chief Murphy Paul told a news conference.

The second officer, Howie Lake, was suspended for three days for failing to maintain his composure. The decisions followed an administrative review of the July 2016 shooting, and both officers plan to appeal, Paul said.

The steps are designed "to bring closure to a cloud that has been over our community for far too long," he said.

Police released four videos of the confrontation with Sterling outside a convenience store, where he was selling CDs.

Paul called the footage from a police dash camera, officers' body cameras, and a store surveillance camera "graphic and shocking to the conscience". Salamoni's camera shows him yelling at Sterling with expletives to put his hands on a car. He points a gun at his Sterling's head and shouts he will shoot him if he moves.

While struggling with Sterling, both officers' cameras came loose. Lake's footage ends by showing Sterling on his back in the parking lot, blood draining from his body.

Sterling, 37, was shot after a resident reported he had been threatened by a black man selling CDs. Police said Sterling was trying to pull a loaded gun out of his pocket when Salamoni opened fire.

Lawyers representing Sterling's five children applauded Salamoni's firing, but expressed disappointment that the officers would not face charges.

"The person who was out of control was Blane Salamoni," attorney Michael Adams said at a news conference. "The person who stood by and let him be out of control was Howie Lake. That's a tragedy." Louisiana Attorney General Jeff Landry said this week that Lake and Salamoni would not face charges since they had reason to believe that Sterling was armed and was resisting arrest.

The US Department of Justice declined to prosecute the officers for civil rights violations in 2017, citing insufficient evidence.