The fire broke out about 2 p.m. and about four hours later, it had surged to 5,000 acres, or nearly 8 square miles, according to the California Department of Forestry and Fire Protection.
TV news cameras have showed at least five structures burning in isolated areas, and at least two of them appeared to be homes, but fire officials would not confirm that homes had burned.
More than 400 firefighters, five helicopters and five air tanker planes were working to get ahead of the flames in Riverside County near Banning.
One firefighter has been injured, but details are unknown.
Mandatory evacuations have been called for the rural communities of Poppet Flats, Twin Pines and Silent Valley, and evacuation centers were set up at high schools in Hemet and Banning.
Margaret Runnels of Poppet Flats was at work when her house came under an evacuation order. She was in Banning waiting for her husband to collect pets and valuables from their house.
"I was hoping they would let me back up to get some personal items I knew my husband would forget like a jewelry box and stuff that means stuff," a crying Runnels told the Desert Sun newspaper. "You always tell yourself to prepare everything but you never take the stupid time to do it."
It wasn't immediately clear how many homes or people were affected by the evacuations.
Temperatures in the area were in the high 80s and low 90s on Wednesday afternoon, with winds blowing up to 20 mph.
State Route 243, a mountain road between Banning and Idyllwild, was closed to non-emergency traffic.
About 30 miles to the southwest, firefighters were working on a 1,100-acre wildfire that has been burning since Monday southwest of Lake Elsinore. That blaze is 78 percent contained and full containment was expected by Friday.
The fire caused evacuations the night it broke out, but the last of them were called off Tuesday night.