Some of the fireworks went into the crowd -- estimated at 8,000 to 10,000 people -- gathered for the Fourth of July display in Simi Valley, the Los Angeles Times reported. Simi Valley Police Cmdr. Blair Summey said fireworks tipped over "like dominoes" after the initial detonation.
The fireworks, sponsored by the Rotary Club at Rancho Santa Susana Community Park, was put on by Bay Fireworks. The New York company has been used by Walt Disney World, NASA and the Republican National Committee.
The audience was at least 800 feet from the explosives, police said.
"There was a big boom. Everybody started running down the street. People were screaming," said Justice Allen, 17, of Simi Valley. "Everybody was just terrified. People hid in bushes."
Another witness, Annisa Wynn, told KTLA-TV, Los Angeles, fireworks began shooting sideways along the ground into the crowd instead of up in the air about 2 minutes into the show.
"The fireworks were so close, you could see them on the ground," Wynn said.
Victims mostly suffered shrapnel-like injuries or burns, the Times said.
Four people with moderate-to-severe injuries and 16 people with minor injuries were taken to a hospital, the Times said. The others were treated at the scene and released.
While there was panic at first, police said the crowd left the park in an orderly way.