By Sharon Bernstein

SACRAMENTO, Calif. (Reuters) - California Governor Jerry Brown on Friday signed a sweeping package of gun control bills, banning high-capacity ammunition magazines and expanding the definition of prohibited assault weapons in the wake of mass shootings in San Bernardino and Orlando.

Democrats in the legislature rushed the measures through in hopes of passing them before their summer break, in part to try to forestall a competing gun control proposal headed for the November ballot.

California already has some of the toughest gun control laws in the nation, but after the shooting spree in the Southern California city of San Bernardino last December, lawmakers began work on measures they said would close unintended loopholes.

"My goal in signing these bills is to enhance public safety by tightening our existing laws in a responsible and focused manner, while protecting the rights of law-abiding gun owners," Brown said in a signing measure.

It was a rare success for advocates of greater gun control.

U.S. lawmakers have fallen short in attempts to tighten gun laws after the killing of 49 people at a gay nightclub in Orlando last month.

Republicans in the U.S. House of Representatives introduced a measure on Friday that would give authorities three days to prove that someone on a terrorism watch list should not be allowed to obtain a firearm. But Democrats rejected it as "toothless."

The bills signed by Brown ban so-called bullet buttons, which allow quick changes in the magazine of a military-style weapon, and require background checks for purchasers of ammunition.

Ammunition magazines that hold more than 10 bullets at a time will also be banned, and background checks will be needed for people borrowing guns from non-family members.

Brown also vetoed several bills, including one that would have allowed co-workers, educators and mental health professionals to request restraining orders forbidding people deemed dangerous from owning guns.

Gun rights advocates called bills a "Gunpocalypse."

"The California Legislature showed their true faces today,” said Craig DeLuz, spokesman for the Firearms Policy Coalition. “They abused the legislative process to enact their depraved anti-civil rights agenda."

Their efforts spilled into intra-party politics as well, after Lieutenant Governor Gavin Newsom, who is expected to run for governor in 2018, gathered enough signatures to place his own gun control referendum on the November ballot.

The move angered the state Senate's top Democrat, Kevin de Leon, who had been working to pass many of the same measures through the legislature.

Worried in part that Newsom's initiative would boost turnout in November among Republicans who oppose gun control, legislative Democrats rushed to pass their bills in time for Newsom to withdraw his measure.

But the lieutenant governor refused.

"Today’s steps in the right direction will grow into a giant leap forward for public safety if voters pass the Safety for All initiative to keep guns and ammo out of the wrong hands," he said.

(Reporting by Sharon Bernstein; Editing by Alistair Bell)