(CNN) California lawmakers proposed a dramatic change Tuesday in the standard under which police officers can use deadly force, in the wake of the fatal police shooting of Stephon Clark.

Joined by the grandfather of the 22-year-old unarmed black man whose death last month has sparked days of protests in Sacramento, legislators announced a bill replacing the current "reasonable force" rule with a stricter "necessary force" standard.

The legislation would authorize officers to use deadly force "only when it is necessary to prevent imminent and serious bodily injury or death -- that is, if, given the totality of the circumstances, there was no reasonable alternative to using deadly force, including warnings, verbal persuasion, or other nonlethal methods of resolution or de-escalation," according to Democratic Assemblywoman Shirley Weber of San Diego, a co-author of the measure.

The bill is known as the Police Accountability and Community Protection Act.

"We're not saying that law enforcement officers can never use deadly force," Weber told reporters in Sacramento. "I want to stress that. Deadly force can be used but only when it's completely necessary."

The proposal also would establish that a homicide by an officer is "not justified if the officer's gross negligence contributed to making the force 'necessary,'" according to the proposal.

Question of 'necessary' or 'reasonable' force

"This reasonable modernization (in the law) focuses on 'only when necessary' as opposed to 'when reasonable,' which is very different in the eyes of the beholder," Assemblyman Kevin McCarty, a co-author, said in a statement.

Lizzie Buchen, a legislative advocate for the ACLU of California, which is pushing the measure, said the current law has long provided "legal cover" for police officers involved in fatal shootings. Under the law, use of force is permitted if officers have a reason to fear for their safety.

"Clearly, the current standard where they can use use deadly force when they feel a serious threat isn't enough to prevent unnecessary deaths of members of community, particularly people of color," she said.

"Deadly force can longer be the first response to a perceived threat. It can only be the last resort."

Proponents of the change called the legislation groundbreaking.

A representative for San Francisco District Attorney George Gascón said the former police chief supports the measure.

Law enforcement organizations have staunchly opposed previous legislative attempts to strengthen the use of force standard.

What two experts say

Leslie McGill, executive director of the California Police Chiefs Association, and Cory Salzillo, legislative director for the California State Sheriffs' Association, said the organizations are reviewing the bill.

Geoffrey Alpert, a professor of criminology at the University of South Carolina and an expert on use of force, said the bill "doesn't seem to change anything other than the language officers will use when explaining why deadly force was used."

"Investigators will use the same criteria provided by the Supreme Court, albeit using different language," he said, referring to two 1980s Supreme Court decisions -- Tennessee v. Garner and Graham v. Connor -- that established the legal framework for determining when deadly force is reasonable.

William Terrill, a professor at Arizona State University's School of Criminology & Criminal Justice, called the legislation "a positive step" toward better police and community relations and "ultimately improved police professionalization."

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"At present, the objectively reasonable standard is simply too open-ended and weighted too heavily in favor of the police," he said.

"An imminent threat should require the suspect actually brandishing or threatening the officer to constitute legal force," Terrill said. "So any language that fleshes out the objectively reasonable standard more concretely is a positive step."

The legislation would make California the first state to restrict deadly force beyond the Graham v. Connor standard, Terrill said.

The lawmakers said police shot and killed 162 people in California last year, only half of whom were armed with guns. Police departments in the state have some of the highest rates of killings by police in the nation, they said.

"Existing use-of-force laws have made an encounter with law enforcement -- no matter how ordinary and no matter whether an individual is unarmed or even cooperative -- into one that ends in the death of a civilian," Weber said in a statement.

Stephon Clark and his family.

"The worst possible outcome is increasingly the only outcome, especially in communities of color."

Clark's shooting death caused outrage among residents who took to the streets to demand the officers be held accountable. Sacramento police have said the officers fired only because they thought their lives were at stake.

Clark was shot by police on the evening of March 18 in his grandmother's yard after they responded to a 911 call about a man who was breaking car windows.

The officers told investigators they opened fired after Clark turned and advanced at them holding what they believed was a gun, police said. A cell phone -- but no weapon -- was found near his body.

Police at the time said officers fired 20 shots at Clark.

An independent autopsy found that Clark was shot eight times , and six of those wounds were in his back, according to Dr. Bennet Omalu, the forensic pathologist retained by Clark's family to conduct a second autopsy.

Clark also received a gunshot wound to his side and another to his left thigh, Omalu said.

Clark's death was "not instantaneous," Omalu told reporters, estimating the father of two died between 3 to 10 minutes after being shot.

"You could reasonably conclude that he received seven gunshot wounds from his back," Omalu said. Each of those bullets possessed "a fatal capacity," he said.

"Meaning that, out of all the seven, all he needed to have died was just one of the seven."

An autopsy conducted by the Sacramento County Coroner determined the cause of death as multiple gunshot wounds, and the manner of death was homicide, according to a preliminary report. But authorities have not released the full report.

The fatal shooting was recorded by two officers' body cameras and from a police helicopter.

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The videos showed a brief encounter between police and Clark, lasting less than a minute, from the moment one of the officers spotted him in the driveway and yelled, "Hey, show me your hands. Stop. Stop."

In the dark, the two officers chased Clark into the backyard of his grandmother's home.

"Show me your hands!" one of the officers yelled. "Gun, gun, gun."

Police then opened fire. Clark crumpled to the ground, momentarily tried to crawl before falling motionless as more shots erupted around him.

California Attorney General Xavier Becerra is holding an independent investigation into the shooting and a review of police procedures.

Two officers -- one of whom is black -- have been placed on administrative leave amid a use-of-force investigation.