Hayward police closed Southland Mall on Saturday afternoon after about 200 teenagers and young adults stormed the shopping center and began ransacking stores.
The crowd poured out of bleachers, set up in the parking lot outside Macy's, when a basketball game known as streetball ended at 5:30 p.m.
Alan Black, who worked on the security team for the sporting event, said the crowd became rowdy after the game.
"People began pushing and just going crazy," he said.
Police Lt. Mark Koller said people entered Macy's and "started tearing the place up." Witnesses reported bands of roving youths pulling clothes off hangers, overturning displays on first-floor sales counters and knocking over mannequins.
Employees and shoppers also saw the vandals kick merchandise and kiosks as they ran through the mall's interior, past Mervyn's and JCPenney stores.
Rick Novac, a 20 20 Fashion store employee, described the crowd as larger than the estimate given by police.
"There were at least 300, easy," he said. "They were running through the mall screaming and kicking things."