Northwest of Los Angeles, the Mountain Fire exploded in size and prompted evacuation orders for more than 10,000 people as it threatened 3500 structures in suburban communities, ranches and agricultural areas around Camarillo, according to a statement from Governor Gavin Newsom.
The area east of the Pacific coast city of Ventura will receive federal assistance after a request from Newsom was granted, the Federal Emergency Management Agency said.
The blaze was burning in a region that has seen some of California's most destructive fires over the years. A thick plume of smoke rose hundreds of feet into the sky on Wednesday, blanketing whole neighbourhoods and limiting visibility for firefighters and evacuees.
It was not known how many properties had been damaged in the wildfire. (AP PHOTO)
Ventura County fire captain Trevor Johnson described crews racing with their engines to homes threatened by the flames to save lives.
"This is as intense as it gets. The hair on the back of the firefighters' neck I'm sure was standing up," he said during a news conference.
At one spot, flames licked the burning remains of a home. Its roof was reduced to only a few charred shingles.
Two people suffered apparent smoke inhalation and were taken to hospitals, fire officials said. No firefighters reported significant injuries.
The erratic winds and limited visibility grounded fixed-wing aircraft, and gusts topped 98km/h, said weather service meteorologist Bryan Lewis. Water-dropping helicopters were still flying.
First responders pleaded with residents to evacuate. Deputies made contact with 14,000 people to urge them to leave as embers spread up to 4km away and sparked new flames.
"This fire is moving dangerously fast," Ventura County fire chief Dustin Gardner said.
Aerial footage from local television networks showed dozens of homes in flames across several neighbourhoods as embers were whipped from home to home. Other footage captured horses trotting alongside evacuating vehicles.
Officials said they were using all resources, including water-dropping helicopters and fixed-wing aircraft dropping fire retardant, but it was still burning out of control.
Andrew Dowd, a Ventura County fire spokesperson, said he did not have details of how many structures had been damaged.
Meanwhile to the south, Los Angeles County Fire Department crews scrambled to contain a wildfire near Malibu's Broad Beach as authorities briefly shut down the Pacific Coast Highway as flames burned near multimillion-dollar properties.
With predicted gusts between 80km/h and 160km/h and humidity levels as low as eight per cent, parts of southern California could experience conditions ripe for "extreme and life-threatening" fire behaviour into Thursday, the weather service said.
