Hundreds of demonstrators have blocked traffic outside a venue in California where Republican presidential hopeful Donald Trump was holding a rally.
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.
Mr Trump has vowed to deport millions of illegal immigrants if he is elected US president in November.
He faces strong opposition in parts of California, particularly among Latinos.
California, the biggest prize for Republican candidates in the nomination race, holds its primaries on 7 June.
On Tuesday, Mr Trump called himself the Republican “presumptive nominee” after five new primary wins in the states of Connecticut, Delaware, Rhode Island, Maryland and Pennsylvania.
‘Build that wall!’
The Pacific Amphitheatre in Costa Mesa was filled to its capacity of about 18,000 people and hundreds more were turned away.
Heated exchanges could be heard between Trump supporters and the protesters outside, with supporters chanting “Build that wall! Build that wall!”, a reference to the candidate’s call for a barrier between the US and Mexico to stop illegal border crossings.
Police in riot gear and officers on horseback moved in to separate the two groups.
Reports from the scene say some protesters threw stones at motorists while others jumped on top of a police car, smashing its windows, the BBC’s James Cook reports from Los Angeles.
The protesters dispersed by 23:00 (06:00 GMT Friday), the Orange County Sheriff’s Department reports.
Mr Trump’s campaign has been dogged by violence between his supporters and protesters, most notably at the University of Illinois in Chicago on 11 March, when a rally was called off after fighting broke out in the auditorium.
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.
In the Democratic camp, Hillary Clinton has a commanding lead over Bernie Sanders.
The presidential election on 8 November will see America vote for a successor to Barack Obama, a Democratic president standing down after two terms in office which have seen the Republicans take control of both houses of Congress.