Police from several cities were called in to quell the disturbance Sunday night in Huntington Beach. Several officers and people received minor injuries.
The weeklong surfing competition drew thousands of people to the Orange County city that bills itself as "Surf City USA."
A large fight broke out between some people in a crowd after the contest ended. When officers tried to break it up, the mob surged downtown, police Lt. John Domingo told KNX-AM on Monday.
The crowd pulled down a stop sign, and a man used it to smash the front window of a bicycle shop, which was then looted. Other people brawled, threw bottles, toppled portable toilets, rocked a city truck and threw a chair into a fountain.
"You see a mob of people running up the street," said Bert Etheredge, a salesman at the Easyrider bicycle and skateboard shop. "That's when we decided to shut down the shop and turn off all the lights, lock everything and kind of go into hiding, if you will."
The crowd then pulled down the stop sign, he said.
"It was coming through the window two seconds later, and that's when people started jumping in and pulling the bikes out," Etheredge told KABC-TV.
Police used nonlethal ammunition to help disperse the crowd.
Witness Doug Cavender told KABC-TV the scene was "really, really nuts, and it got really crazy really fast."
"I really honestly didn't think stuff like this happened in the OC," he said.
One person was arrested on suspicion of assault with a deadly weapon, and seven were arrested for other crimes including disturbing the peace, unlawful assembly and inciting a riot, Domingo said.
"We're extremely disappointed and saddened by the disturbance," said a statement on the U.S. Open of Surfing website. "We work tirelessly with city staff, police, fire and other agencies to ensure a safe environment for all."
Investigators planned to review video and could make more arrests.