The crowd gathered in the streets outside the OC Fair & Event Center as Trump addressed several thousand supporters at the Center's amphitheater. At least one police car was damaged and several scuffles broke out amid the hectic scene.
Protesters blocked a main intersection, impeding traffic, and officers with the Orange County Sheriff's Department and Costa Mesa Police Department worked to disperse the crowd, ordering protesters out of the streets.
About 20 people were arrested, the Orange County Sheriff's Department tweeted late Thursday night after the protests had cleared.
Lt. Mark Stichter, the Sheriff's Department's public information officer, could not provide an official estimate on the number of protesters, but demonstrators could be seen filling the intersection of Fairview Road and Fair Drive.
Several scuffles broke out between Trump supporters who were leaving the rally and people in the streets who accused them of being racists. One Trump supporter was visibly bloodied after being punched in the face.
Several people damaged a police car, smashing its back window before jumping on it and kicking its doors. As a crowd formed around the car, police officers in tactical and riot gear moved into action, forming a perimeter around the crowd before forcing the demonstrators to move down the road.
While some demonstrators shouted insults and slurs at police officers, others focused on delivering a message of protest against the Republican front-runner's rhetoric.
Several protesters told CNN they were demonstrating against Trump's rhetoric on illegal immigration. Some were seen carrying Mexican flags as they marched in the street. Other demonstrators shouted insults and slurs at police officers.
Rojelio Banuelos, a 26-year-old student, carried a sign that read, "Liberation not deportation."
"I'm against Trump's nativist and nationalistic agenda, which divides people and is very hateful of the other," he said.
While Banuelos simply marched through the streets, he called the property damage and anger some demonstrators expressed Friday night "the symptom of hate speech" and said he did not believe any individuals were taking advantage of the protest.
Katie Brazer, a 26-year-old community organizer who marched alongside Banuelos, agreed.
"I think people are tired of these messages of hate," she said.