Firefighters warned that the fire was moving so fast they were unable to contain it.
The blaze started on Monday evening and by early Tuesday had burned 26,000 acres (10,500 hectares).
One person died in a traffic accident while trying to flee the fire, officials said.
More than 260,000 customers were said to be without power.
Fairground open for evacuees
Multiple structures, including a large apartment complex, were ablaze as the fire swept into Ventura city, the LA Times reports.
Emergency workers were going house to house urging people to leave their homes, while hundreds of firefighters worked to protect homes and buildings.
But Ventura County Fire Chief Mark Lorenzen said: "The prospects for containment are not good. Really mother nature is going to decide."
Evacuation centres have been opened in schools and fairgrounds.
The fire is believed to have broken out close to Thomas Aquinas College in Santa Paula, some 50 miles (80km) north of Los Angeles, early on Monday evening.
It was quickly fanned by gusts of up to 70mph (115 kph).
The college said that all its 350 students and staff had been evacuated.
California has been hit hard by wildfires in recent months.
At least 40 people were killed when fires ripped through parts of northern California's wine region in October. Some 10,000 structures were destroyed.
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