Hundreds of people attended a fireworks show sponsored by the Irvine Police Association at Irvine High School Monday night.
The Orange County Fire Authority said an ember from the fireworks sparked the blaze minutes after the show began. Some people thought the fire was part of the show.
The fireworks show was halted so fire crews could battle the blaze. Thousands were evacuated from the stadium. No one was injured.
YouTube video shows the flames intensifying as the stadium lights were turned on.
"I've never seen anything like this happen before, where a fire starts from an aerial firework, from a professional shoot," said Greg McKeown, Orange County Fire Authority. "Even though we had a small mishap at a fireworks display, they are safe, and much safer than lighting fireworks off yourself."
An even bigger fire broke out at Laguna Niguel Regional Park Monday night. That blaze took firefighters more than an hour to extinguish and caused the evacuation of 16 homes. It burned a total of 6 acres before it was knocked down around 10:15 p.m.
Investigators said it's not clear if the fire was caused by the sanctioned public fireworks display at the park or by nearby personal fireworks, which are illegal.
In Redlands, dozens of 911 callers reported illegal fireworks. Fire officials said that may be the cause of a palm-tree fire. The fire department had to return to the same area less than an hour later when the tree re-ignited.
Illegal fireworks also are being blamed for a half-acre fire in the Chatsworth area near Stoney Point, a popular trail for hikers. In that incident, witnesses said a man and a woman were seriously injured by the blaze, but fire crews couldn't locate them.
The threat of illegal fireworks prompted the Los Angeles Fire Department to temporarily beef up its staffing to many engines that had been cut for budgetary reasons.