Newly released video footage of police killing an unarmed black man in a barrage of gunfire in Sacramento, California, left the mayor "horrified" and sparked street protests on Thursday that gridlocked traffic in the state capital for hours.

More than 200 demonstrators gathered on Thursday to denounce the shooting in a protest organised by the Black Lives Matter movement. The protests were orderly and police allowed a group of demonstrators to march onto Interstate 5 at the height of the commuter rush hour as they blocked all southbound lanes.

Police said the video, captured by body cameras worn by officers involved in Sunday night's confrontation, showed the victim, Stephon Clark, 22, holding an object that later turned out to be a cell phone as he was shot 20 times.

The video was released by police late on Wednesday and it soon went viral on the internet.

Sunday's shooting was the latest in a series of killings of unarmed black men by police across the United Stated since 2014 that has sparked a national debate about racial bias in the criminal justice system and the use of lethal force.