Demonstrators swarmed outside the hotel near San Francisco airport, forcing Trump to crawl under a fence to enter the hotel where he met with local GOP power brokers and gave a lunchtime speech at the state party's convention. On Thursday night, protesters tangled with authorities and damaged police cars after a Trump rally in Orange County.
Tensions mounted as the GOP presidential contest moves into its final stages in one of the nation's most liberal and diverse states. The state party convention amounts to the starting bell in California primary, with Ohio Gov. John Kasich appearing later Friday and Texas Sen. Ted Cruz and his new running mate, Carly Fiorina, up Saturday.
During his speech at the convention, Trump called for the party to unify behind him but also issued a veiled threat to its leaders. "There has to be unity in our party," he said. "Would I win - could I win - without it? I think so because they're going to be voting for me" - and not the party, he added.
California's primary usually comes after the party nominees have been decided - but this year it looms as a decisive contest that could either clinch the prize for Trump or force him into a contested convention in July.
All three candidates are looking to galvanize supporters, sway undecided party members or poach from rival campaigns at the convention. "It's going to be a free-for-all," predicted the state party vice chairman, Harmeet Dhillon.
That label clearly applied to Trump's Orange County rally Thursday night, which filled the Pacific Amphitheatre to its capacity of about 8,000, with many hundreds more turned away.
WATCH VIDEO: Car does donuts around Donald Trump protesters
Protests that stayed mostly peaceful during the event grew in size and anger afterward. Police in riot gear and on horseback pushed the crowd back and away from the arena; one Trump supporter had his face bloodied in a scuffle as he tried to drive away. One man jumped on a police car, leaving its front and rear windows smashed and the top dented, and other protesters sprayed graffiti on a police car and the venue's marquee.
About 20 people were arrested, said the Orange County Sheriff's Department.
On Friday, hundreds of demonstrators pushed to the front doors of the Hyatt Regency in Burlingame before being moved back by police in riot gear. Some protesters infiltrated the hotel building and hung a giant banner reading, "Stop Hate."
The Secret Service led Trump in through a rear entrance to the hotel. Trump joked that as he crawled under a fence to get in, he "felt like I was crossing the border."
Trump's remaining rivals can't beat him in what's left of the primary season. Their only hope is to deny him a majority of delegates heading into the July convention and wrestle for the prize in multiple ballots there.
But questions persist in the party - nationally and in California - about Trump's electability in the fall and his conservative credentials. So the reception Trump receives from the state's party activists and grassroots organizers will be noteworthy. He rarely speaks to Republican establishment groups, and he rails against what he calls a rigged party system that governs the nomination.
The California convention crowd defies expectation in a state known as a Democratic fortress. There have been pushes toward moderation, but the group leans conservative and favors calls for free markets, tax cuts and smaller government. It's also socially conservative: The state party's platform defines marriage as between a man and a woman, and wants the Supreme Court's affirmation of abortion rights reversed.
Trump has spoken favorably about Planned Parenthood, which provides abortion services. He has warned against cutting Medicare, Medicaid and Social Security, often targets for conservatives who want to slow government spending.
When Trump earlier this month said transgender people should be able to use whichever bathroom they choose, Cruz's campaign released a statement saying Trump was "no different from politically correct leftist elites." The California platform endorses free markets; Trump has long criticized U.S. trade policy and advocated steep tariffs on Chinese goods.
The California primary will award 172 delegates. Trump now has 996 delegates, Cruz has 565 and Kasich has 153, according to the AP's delegate count. It takes 1,237 to clinch the nomination.