The Cranston fire which started in the San Jacinto Mountains east of Los Angeles on Wednesday has spread to 7.5 square miles.
Five homes have been burnt and evacuation orders are in place for an entire forest town as California swelters under a heatwave.
The blaze is believed to have been started by arson and a man has been charged on suspicion of five counts of arson, state fire officials said.
The Cranston fire was the largest of at least five in the state of California.
Cal Fire said the Cranston Fire, which is burning about 110 miles (177km) east of Los Angeles, had blackened 11,500 acres (4,650 hectares) and was just three percent contained.
The fire which has torched timber and tinder-dry brush has spread to 11,500 acres on its third day of devastation amid more hot, dry weather conditions.
Authorities ordered residents to leave Idyllwild and several neighbouring communities, which are home to about 12,000 people.
The Riverside Sheriff’s station was conducting evacuations of Idyllwild, the Apple Canyon area, the Lake Hemet area, Mountain Center community, the Hurkey Creek area and Camp Scherman Girl Scout Camp.
William Blodgett of Idyllwild called the scenes “apocalyptic” and told KNBC-TV he could not get home because of the fire.
He said: “We were all peeling out of there as fast as we could. It was apocalyptic.”
The entire community of Idyllwild, a mountain town about 100 miles east of Los Angeles and other areas of the San Jacinto Mountains were evacuated on Wednesday evening.
The fire which is devastating the San Bernardino National Forest sent up a cloud of 50,000 feet high which was so enormous it created its own weather in the form of lightning, the National Weather Service reported.
Helicopters and planes have dumped water and fire retardant which has turned swathes of land and homes pink.
Lyn Sieliet, a United States Forest Service spokeswoman said: “It’s hot, it’s dry and the terrain is really steep.
“The humidity is low and there’s a light wind, so all of those conditions contribute.”
The fire is just one of several across California amid a heatwave which has seen temperatures reach the extremes of the high 30s.
Cal Fire officials have said that dry grass and brush in steep terrain was fuelling the blaze.
The fire has prompted a large regional response, including 180 personnel from agencies such as the Contra Costa County Fire Protections District among other local agencies.
Ms Sieliet said the fire had not caused injuries to civilians or emergency services.
On Thursday, Governor Jerry Brown of California had declared a state of emergency in Riverside County.
The declaration also applied to Shasta County in Northern California where a 20,000-acre fire is blazing.