Approximately 20 people were arrested following a Donald Trump campaign event in Costa Mesa, California, last night, according to police, after peaceful protests in conservative Orange County turned violent.

Hundreds of young demonstrators—waving Mexican flags and posing for selfies, the Associated Press reports—blocked cars trying to leave the Pacific Amphitheatre event, provoking confrontations with Trump supporters.

“We’re going to stop drugs from coming in,” Trump had told his supporters inside the arena. “The drugs are poisoning our youth and a lot of other people.”

He also repeated a long-debunked myth about General John Pershing shooting Muslims with bullets dipped in pig’s blood.

The event was Trump’s first in California since February, as the campaign ramps up for the June 7 primary. From the Los Angeles Times:

Orange County is a major target for Republicans. It remains the state’s biggest GOP stronghold, but it’s less conservative than it was in the days when it anchored the careers of Richard Nixon and Ronald Reagan.

Orange County’s population has diversified, with an influx of Asian and Latino residents slowly diminishing the political clout of whites.

A decade ago, Republicans were 48% of Orange County’s registered voters; now they are 40%. Democrats have risen from 30% to 32%, while nonpartisans grew from 18% to 24%.

At least one Trump supporter was left bloodied after an altercation with protestors. Another man smashed in the windows of a police car.

“Build the wall!” Trump supporters chanted at a man who’d draped himself in the Mexican flag, the Times reports. “I love Trump,” retired carpenter Brent Fisher said. “He’ll stand up and fight and do the things he’s talking about.”