Californians will vote in November on far-reaching new restrictions on firearms, including the nation’s first requirement of background checks for buyers of ammunition, Lt. Gov. Gavin Newsom said Thursday.

Declaring a “historic opportunity to recapture our leadership nationwide on this issue,” Newsom said supporters will submit 600,000 signatures on petitions to qualify an initiative that would strengthen California’s gun-control laws, already some of the strictest in the nation. They need 365,880 signatures of registered voters to make the ballot.

Besides the checks for ammunition purchases — like those already in place for guns — the measure would ban possession of large-capacity rifle magazines, require gun owners to notify police when their weapons are lost or stolen, and enact rules for courts to confiscate guns from criminals who are prohibited from possessing them. Other provisions would reclassify possession of a stolen gun as a felony and require California to share its background check information with the FBI.

“We have a chance to vote on the most comprehensive and significant gun-safety measure in decades,” said Newsom, who is sponsoring the initiative while preparing to run for governor in 2018.

Brad Chase, spokesman for the Coalition for Civil Liberties, which is fighting the measure, said top law enforcement officials across the state are overwhelmingly opposed to Newsom’s initiative.

“Representatives for the state’s 58 sheriffs and thousands of prosecutors have written Newsom in protest,” he said. “Our frontline law enforcement is screaming that this is going to criminalize the law-abiding and clog up the court system without doing anything to stop criminals or terrorists.”

The last gun-control measure on the California ballot, a 1982 initiative that would have tightly limited the purchase of new handguns, was defeated by more than a 3-2 majority after an intense and well-funded opposition campaign by the National Rifle Association.

Asked about Newsom’s new initiative, NRA spokeswoman Amy Hunter said Thursday that none of the measure’s provisions would promote public safety.

“We will do everything in our power to put an end to this,” Hunter said. But she wouldn’t say whether the organization plans to spend large sums on the campaign.

“Newsom would like to see guns confiscated in California,” Hunter said.

Newsom called that assertion “fictional.”

One part of the initiative, however, would take away one type of firearm, rifles with magazines that can carry more than 10 cartridges, from people who now can legally possess them in most parts of the state. California bans the sale of such weapons but not their possession, though some cities, including San Francisco and Oakland, prohibit possession as well. Federal courts have upheld the local ordinances.

“I don’t know a legitimate hunter who needs a 15-round clip,” said Newsom, who noted that such weapons are often used in mass shootings.

He said the proposed background-check requirement for ammunition purchases was “potentially a game-changer in the gun-safety debate.”

Newsom said a convicted felon who had illegally obtained a gun now “could go down and purchase an unlimited amount of ammunition” from “anyone with a business license,” with no questions asked. He said the initiative would require dealers to be licensed, forbid sales of ammunition to those with histories of serious crimes or mental illness, and — as with guns — require all sales to be face-to-face.

The initiative has been endorsed by the California Democratic Party, Sen. Dianne Feinstein, and Mayors Ed Lee of San Francisco, Libby Schaaf of Oakland and Sam Liccardo of San Jose. Newsom said supporters had spent more than $3 million for signature-gathering and are prepared to raise whatever they need to win in November.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @egelko