The Los Angeles Times reported most of the arrests occurred after a female demonstrator allegedly attacked a television videographer and police ordered the crowd of about 150 people to disperse.
KCBS-TV said the videographer was struck in the face and sought shelter in the TV station's van while several protesters surrounded the vehicle.
"We have bent over backward today to allow them to protest, hoping to keep it peaceful," police Sgt. Jeff Stuart said.
The protesters who gathered outside the Fullerton Police Department demanded justice for Kelly Thomas, who was fatally beaten last July. A jury Monday found former Fullerton officers Manuel Ramos and Jay Cicinelli not guilty in the homeless man's death.