Lt. John Domingo tells KNX-AM a large fight broke out, and when officers tried to break it up, the mob surged downtown.
A bicycle shop was looted, and TV reports showed people brawling, overturning a portable toilet, rocking a truck and throwing a chair into a fountain.
Police from several cities were called in. Domingo says arrests were made for disturbing the peace, unlawful assembly and inciting a riot. One person was held for assault with a deadly weapon on an officer.
Police will look at video, and more arrests could be made.
The weekend surfing competition drew thousands of people to Huntington Beach, a popular surfing spot.
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