Mr Trump brought his campaign to conservative Orange County after sweeping the northeast Republican primaries.
Dozens of protesters were mostly peaceful on Thursday as Mr Trump gave his speech inside the Pacific Amphitheatre. After the event, however, the demonstration grew rowdy late in the evening and spilled into the streets.
Approximately 20 people were arrested by Costa Mesa police, according to a tweet from the Orange County Sheriff’s Department. One Trump supporter had his face bloodied in a scuffle as he tried to drive out of the arena. One man jumped on a police car, leaving its front and rear windows smashed and the top dented. Other protesters sprayed graffiti on a police car and the venue’s marquee.
Dozens of cars – including those of Trump supporters trying to leave – were stuck in the street as several hundred demonstrators blocked the road, waved Mexican flags and posed for selfies.
Police in riot gear and on horseback pushed the crowd back and away from the venue. There were no major injuries and police did not use any force. The crowd began dispersing about three hours after the speech ended.
Earlier in the evening, half a dozen anti-Trump protesters taunted those waiting to get into the venue.
Mr Trump’s supporters surrounded a man who waved a Mexican flag and shouted “Build that wall! Build that wall!” – a reference to the presidential hopeful’s call to create a barrier between the US and Mexico.
Seven women wearing no shirts and Bernie Sanders stickers over their breasts entered the square outside the amphitheatre. They said they were protesting over Mr Trump’s lack of engagement on gender equality and women’s rights. “I feel like he wants to make America great again, but certainly not for women” said one of the women, Tiernan Hebron.