Hundreds of people rallied for Stephon Clark, a 22-year-old who was shot Sunday in the backyard of his grandparents' home. Police said they feared he had a handgun when they confronted him after reports that he had been breaking windows in the South Sacramento neighbourhood.

But police found only a mobile phone.

"We are at a place of deep pain" because of recent violence directed at black people in Sacramento and elsewhere, said the Rev. Les Simmons, a community leader.

Sacramento Mayor Darrell Steinberg earlier said he was horrified but wouldn't second-guess the "split-second decisions" of the officers.

He said the department had improved its policies since the fatal shooting of a mentally ill black man in 2016.

But independent experts said the footage from body cameras and an overhead helicopter raised more questions than it answered.

The officers appeared to believe they were in danger, they said, and if so the shooting was likely legally justified.

The Sacramento Police Department said officers were responding to reports of a man seen breaking into at least three vehicles and later into a neighbour's home.

The police said deputies in the helicopter saw Clark break a neighbour's sliding glass door before jumping a fence.

As a result, "their threat radar is really high," said Plumas County sheriff's deputy and special prosecutor Ed Obayashi, who trains officers and testifies in court on police use of force.

"They have to assume that their lives are in danger at that very second," he said.

Australian Associated Press