The blaze, dubbed the Thomas Fire, broke out on Monday evening in the foothills above Ventura. Winds quickly drove it west into the city some 50 miles northwest of Los Angeles.
By Tuesday evening it remained zero percent contained, and had charred more than 50,000 acres, fire officials said.
California Governor Jerry Brown declared a state of emergency, freeing state funds and resources to assist the more than 1,000 firefighters battling to save homes from the conflagration.
"This fire is very dangerous and spreading rapidly, but we'll continue to attack it with all we've got," Brown said in a statement. "It's critical residents stay ready and evacuate immediately if told to do so."
There were no immediate reports of fatalities from the blaze but KABC-TV reported that one person was killed in a car crash while fleeing the area.
More than 250,000 homes were without power, utilities said. All schools in the Ventura Unified School District were closed.
In the San Fernando Valley north of Los Angeles, the so-called Creek Fire had blackened more than 11,000 acres and forced the evacuation of 2,500 homes and a convalescent center north of Interstate 210. The highway remained open even as other roads were closed, officials said.
Los Angeles Mayor Eric Garcetti declared a state of emergency in the city.