Donald Trump supporters leaving the presidential candidate's rally in San Jose were pounced by protesters, some of whom threw punches and eggs.
A dozen or more people were hit and car windows were broken. Trump hats grabbed from supporters were set on fire on the ground. One least one woman was pelted with an egg.
Police stood their ground at first but after about 90 minutes moved into the remaining crowd to break it up and make arrests. At least four people were taken into custody, though police didn't release total arrest figures Thursday night. One officer was assaulted, police Sgt. Enrique Garcia said.
There were no immediate reports of injuries and no major property damage, police said.
The crowd, which had numbered over 300 just after the rally, thinned significantly as the night went on, but those that remained near the San Jose Convention Center were rowdy and angry.
Some banged on the cars of Trump supporters as they left the rally and chased after those on foot to frighten them.
Police were keeping their distance from the crowd as the scuffles played out, but keeping them from getting any closer to the convention center.
"Our police officers have done an extremely courageous and professional job so far," San Jose Mayor Sam Liccardo told The Associated Press by phone. "We're all still holding our breath to see the outcome of this dangerous and explosive situation."
The mayor, a Democrat and Hillary Clinton supporter, criticized Trump for coming to cities and igniting problems that local police departments had to deal with.
"At some point Donald Trump needs to take responsibility for the irresponsible behavior of his campaign," Liccardo said.
Such protests are unlikely to be repeated on Friday when Trump holds a rally at the airport in the much smaller and more conservative city of Redding about 150 miles north of Sacramento.
Clinton and Democratic opponent Bernie Sanders will also make campaign stops in the state on Friday in California as they look to Tuesday's California Primary.
Trump, the presumptive GOP nominee, spoke for about 50 minutes at the rally, sniping at Democratic candidate Hillary Clinton and calling her speech on foreign policy earlier in the day "pathetic" and "sad to watch."
Protesters before the speech included Adam Rivas, a 22-year-old community college student who was born and raised in San Jose, was holding a spray-painted sign that read "Dump Trump."
Rivas said he was particularly disturbed by Trump's remarks about Mexicans.
"For any one Mexican here he bashes, there are about 20 Mexicans out there who are hard-working and just doing their job," he said.
Trump supporter Debbie Tracey, a U.S. Navy veteran from San Jose, she came to hear Trump speak, she left his rally with two hats a T-shirt and a handful of signs that said "Veterans for Trump."
Passing in front of a wall of protesters, many chanting in Spanish, she said she supported Trump's call for a wall along the U.S.-Mexico border.
"I'll go help build the wall because if you are going to come to this country, land of opportunity, you should be here legally," she said.
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