Protests erupted in California for the second day in a row on Friday against US presidential candidate Donald Trump, who is moving closer to winning the Republican nomination after a string of victories this week.

The billionaire businessman was forced to halt his motorcade and go through a back entrance to a hotel to give a speech to the California Republican convention and avoid several hundred loud protestors gathered outside.

"That was not the easiest entrance I've ever made," Trump told the gathering in Burlingame, south of San Francisco, after weaving around a barrier and clambering across a road to get to the venue. "It felt like I was crossing the border actually."

Demonstrators, some of whom held Mexican national flags, at one point rushed security gates at the hotel and police officers had their batons out.

The mogul had already drawn protests in California, with chaotic scenes on Thursday outside a Trump rally in Costa Mesa. Anti-Trump protesters smashed the window of a police car and blocked traffic. Some 20 people were arrested.

Protests have become common outside rallies for Trump who has earned ardent critics, as well as support from Republican voters, for his rhetoric against illegal immigration. His campaign abandoned a rally in Chicago last month after clashes between his supporters and protesters.

He has accused Mexico of sending drug dealers and rapists across the US border and has promised to build a wall and make Mexico pay for it.

Trump, who described himself this week as the party's presumptive nominee, would take a large stride toward knocking his Republican rivals out of the presidential race if he wins the Indiana primary next week.

On Friday, he said he is approaching the 1,237 delegates needed to clinch the nomination.

Trump, who has run as a political outsider and only recently started making inroads with the Republican establishment, called for the party to band together behind him. But said he could win the White House without them if needed.

"There should be and there has to be unity. Now with that being said, would I win, can I win without it? I think so, to be honest," Trump told the convention. His speech drew applause, though not the fervent reception of his usual campaign rallies.

INDIANA FIGHT

Trump's main rival, Senator Ted Cruz, on Friday picked up the backing of Governor Mike Pence of Indiana in a rearguard battle to damage Trump's chances.

"I'm not against anybody, but I will be voting for Ted Cruz in the upcoming Republican primary," Pence said on an Indiana radio show.

Cruz, from Texas, is trailing the former reality TV star in the Midwestern state after losing to him by a wide margin in all five Northeastern states that held nominating contests on Tuesday.

A CBS poll earlier this week found Trump with about 40 percent of support in Indiana, compared to 35 percent for Cruz. The poll had a margin of error of 6.6 points. Other polls have also shown Trump ahead.

The Republican front-runner was in California ahead of its June 7 primary, when the most convention delegates of the Republican nominating cycle will be at stake.

After his speech, Trump made a similarly unconventional exit out of the hotel via the back door.

Cheryl McDonald, 71, of Discovery Bay, said she had to pass through protesters to get inside the hotel.

"They were yelling. I think the only words they know in the dictionary are profanities," said McDonald, who said she is a Trump supporter.

Ohio Governor John Kasich, a distant third in the race for the party's nomination, distanced himself from what he said was a divisive campaign that preyed on voters' fears.

"I'm worried about a divided, polarized country," Kasich said. "It doesn't have to be that way."-Reuters