Several thousand homes are under mandatory evacuation in the cities of Ventura and Santa Paula, some 70 miles (115 km) north of Los Angeles.
Firefighters warned the fire was moving so fast they were unable to contain it.
Fanned by high winds, the fire swept through tens of thousands of acres in a matter of hours.
California Governor Jerry Brown has declared a state of emergency in Ventura County, promising to attack the fire "with all we've got".