COSTA MESA, Calif. — Protesters opposing Donald Trump surged against riot police and taunted the candidate's supporters Thursday as a campaign rally became a tense political confrontation that resulted in about 20 arrests.

There were no reports of serious injuries at the event in Republican bastion of Orange County. But at least two police cars were vandalized, and one Trump supporter had his face bloodied in a scuffle as he tried to drive away from the arena after Trump's speech.

The fracas marked the most recent flash of tensions that have followed Trump to campaign events around the country. Hundreds of protesters gathered outside the Orange County Fair & Event Center after Trump finished his speech, chanting and waving signs insulting the candidate. Some demonstrators waved Mexican flags in apparent opposition to Trump's call for a border wall.

A Twitter message by the Orange County Sheriff’s Department said about 20 people were arrested, but the possible charges were not immediately clear.

The situation escalated when a protester smashed the back window of a police cruiser. Soon after, someone jumped onto the vehicle and aggressively stomped on the top. Others began to jostle the car back and forth as people in the crowd egged on the action.

Other protesters spray-painted graffiti on a police car and over the marquee at the event center, the Associated Press reported.

Police, including units on horseback, pushed back the line of protesters. Some protesters blocked traffic and shouted insults at presumed Trump backers.

Earlier, the Associated Press reported, Trump supporters surrounded one man carrying a Mexican flag and shouted: “Build that wall! Build that wall!”

“Let’s go! Go home!” police officers shouted, continuing to push the line forward.

“They want to batter us, they want to oppress us. F--- that s---,” yelled one young woman as the crowd was moved along. The crowds dispersed after about three hours.