US election 2016: Clashes near Trump rally in California
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.
Mr Trump has called himself the Republican "presumptive nominee" after a string of primary wins.
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.
They dispersed by 23:00 (06:00 GMT Friday), the Orange County Sheriff's Department reports.
The candidate did not seem fazed by the clashes, tweeting after the rally: "Thank you Costa Mesa, California! 31,000 people tonight with thousands turned away. I will be back! #Trump2016"
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.
Meanwhile, a detailed profile of Mr Trump's wife Melania for GQ Magazine has sparked a hostile response from her.
The article revealed details about her upbringing in Slovenia and how she met Mr Trump. She called it "disingenuous reporting".
Julia Ioffe, the Jewish Russian-American reporter who wrote the article, said on Twitter she has received a barrage of anti-Semitic and threatening messages from Trump supporters since the piece came out.
"Now I'm getting phone calls from a blocked number that play Hitler's speeches when I pick up. Sad!" she tweeted.
"It's unsettling," she told the Guardian.
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.