Hundreds of demonstrators blocked a police van and then entered the parking area of the San Jose Convention Center where the event took place, insulting Trump supporters in their cars and attacking those leaving the rally.
A dozen or more people were punched, the Los Angeles Times reported, saying at least one person was hit with an egg.
ABC News reported that some protesters surrounded the car of a presumed Trump supporter as it left the convention center garage, shaking the car and smashing its tail light.
Crowds had earlier chanted “No hate in our state” and carried signs that read “Dump Trump” as they marched near the convention center.
One protester was seen burning an American flag and some supporters were spat on. Several fistfights erupted but were quickly broken up.
Police were out in force, at one point forming a line to allow Trump supporters access to the rally and prevent clashes that have erupted at a number of the real estate magnate’s events in recent weeks.
Trump was last in the San Jose area in April, when he spoke at the California Republican Party convention where protesters blocked access to the venue, forcing him to use the back door.
Last week, dozens of protesters were arrested in San Diego after a campaign appearance by Trump, who has angered many with his rhetoric against Hispanics, women, Muslims and others.
Demonstrations at a Trump rally in New Mexico last month also turned violent as protestors overran barricades and clashed with police in riot gear.