Southern California's 2004 fire season opened Monday with firefighters battling blazes that covered nearly 5,000 acres and forced hundreds of people to evacuate.

The largest was a 2,500-acre fire in Riverside County (search). But five other blazes were scattered across Santa Barbara, Los Angeles and San Diego counties, as temperatures climbed to 100 degrees in some places.

Several firefighters among the thousands on the job were treated for injuries, including heat stroke, dehydration and smoke inhalation.

Authorities ordered a mandatory evacuation of several homes in the path of the Riverside County fire in the hills south of Temecula (search). They previously advised residents of about 100 homes to evacuate.

The blaze destroyed a home and a research facility used by researchers at the University of California, Riverside.

"They are just totally burned to the ground. All that's left is a couple of pieces of steel," said Rick Cook, emergency services coordinator for Riverside County.

Another Riverside County fire that broke out Monday afternoon in El Cerrito spread across 400 acres in less than three hours, said Department of Forestry spokeswoman Becky Luther. She said it threatened up to 30 homes.

A fire in the Los Padres National Forest (search), in Santa Barbara County, grew to about 100 acres within two hours.

Firefighters had about 75 percent containment on a 50-acre blaze that broke near a Los Angeles County jail in Castaic, said fire Capt. Mark Savage.

Firefighters said they managed to slow a blaze that burned 1,500 acres near the Camp Pendleton (search) Marine base in San Diego County. It was about 45 percent contained, with no structures damaged.

A fire that started Sunday just east of Lake Elsinore in Riverside County had scorched 350 acres. One structure was burned.

The start of the fire season was declared three weeks earlier than last year because of dry weather and a tree-killing bark beetle infestation.