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.
"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.
The Los Angeles Times reported that a car hit a firefighter who was protecting homes from the flames. It said he was being treated in hospital.
More than 250,000 homes were without power, utilities said. All schools in the Ventura Unified School District were closed.
The Thomas Fire was the largest of several large blazes that broke out across Southern California following the onset of the Santa Ana winds.
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 centre 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.
"We're chasing the fire, trying to get ahead of it, trying to get in front to provide structure defence," Los Angeles County Chief Deputy David Richardson told reporters at an afternoon briefing as thick black smoke drifted across the city.
The Santa Ana winds, which blow in from the California desert, were forecast to top out at 70 miles per hour (115 km per hour) and remain strong through the week.
(Additional reporting by Peter Szekely in New York, Brendan O'Brien in Milwaukee and Dan Whitcomb in Los Angeles; Writing by Dan Whitcomb; Editing by Jeffrey Benkoe and Grant McCool)
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)