Hundreds of protesters broke through barricades and threw eggs at police outside a California hotel where Republican Party front-runner Donald Trump was due to address the state’s Republican convention.
The demonstrations in the city of Burlingame temporarily delayed a speech by the billionaire businessman.
Because of the protest, Mr Trump had to enter the hotel via a rear entrance.
Mr Trump’s rallies have been dogged by violence.
On Thursday, a police car had its windows smashed as Mr Trump spoke inside a hall in the Orange County Fairgrounds in Costa Mesa. Some 20 arrests were made.
The Trump campaign had to cancel several rallies in March after hundreds of protesters threatened to disrupt events in Chicago and St Louis.
Before his speech on Friday, news helicopters showed Mr Trump and his security detail crossing a motorway before entering the hotel via a side door.
On stage, Mr Trump joked about the protesters, saying “that was not the easiest entrance I ever made.”
“I felt like I was crossing the border,” he said, and that he walked through “dirt and mud” to get to the building outside of San Francisco.
Many of the protesters outside his speech were arguing against his positions on immigration. He has advocated building a border wall with Mexico which he says Mexico would pay for.
He has also referred to Mexicans as “rapists” and criminals responsible for bringing illegal drugs into the US.
Mr Trump is extremely unpopular among Latino voters and California has a large Mexican-American population.
Protests are expected to continue until the California primary is held on 7 June.
Mr Trump has called himself the Republican “presumptive nominee” after a string of primary wins.
In terms of delegate support, the property tycoon is far ahead of his nearest rivals, Texas Senator Ted Cruz and John Kasich, the governor of Ohio.