"It's an open-ended question," Governor Gavin Newsom said after adding his signature to the bill, in front of a crowd of legislators, family members of those killed in police shootings and advocates, many of them black or Latino.

"This is remarkable to get to this moment on a bill that was so controversial, but it means nothing unless we make this moment meaningful."

Supporters and law enforcement officials said the new standards, which take effect January 1, are among the nation's most comprehensive when combined with more police training.

But they must be coupled, Newsom said, with cultural and systemic changes including more transparency and a rebuilding of trust with the community.

California's old standard was based on the doctrine of "reasonable fear," meaning if prosecutors or jurors believed officers had a reason to fear for their safety, they could use lethal force.

The new law will allow police to use deadly force only when "necessary" to defend against an imminent threat of death or serious injury to officers or bystanders.

It passed with bipartisan support after major police organisations won concessions and ended their vehement opposition.

Lawmakers dropped an explicit requirement that officers try to de-escalate confrontations. Law enforcement officials said that would have opened officers to endless second-guessing of what are split-second, life-and-death decisions.

The bill's lead author, Democratic Assemblywoman Shirley Weber of San Diego, said the major elements of the bill "are still there, and they will make a difference in California and the nation."

Australian Associated Press