More than a dozen wildfires stoked by powerful winds have swept through California, destroying at least 1,500 homes and businesses and sending an estimated 20,000 people fleeing to safety.

One person was killed in a fire in Mendocino County, further north in the US state, on Monday.

California's fire chief called the damage estimates in the wine country conservative and said the fires were burning throughout an eight-county expanse of Northern California, including Napa, Sonoma and Yuba counties.

Numerous people had been hurt, and some were missing, although no estimates were immediately available, said Ken Pimlott, California Department of Forestry and Fire Protection director.

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Later he said there were probably fatalities.

Mandatory evacuations were ordered after the blazes broke out late on Sunday. Long lines formed at petrol stations when many families heeded a middle-of-the-night call to get out.

A spokesperson for Pacific Gas & Electric said 114,000 customers were without power.

Routes blocked

With fallen trees or flames blocking routes, Sonoma County residents struggled to figure out what roads to take.

Fires also burned just to the east in Napa County as well as in Yuba, Butte and Nevada counties - all north of the state capital, Sacramento.

Many of the fires spread suddenly overnight, whipped by strong winds.

Santa Rosa, the largest city in the fire area with a population of about 175,000, was hit hard.

The city lost a big-box store and unknown numbers of other businesses and homes, as the blaze shut down its schools and forced patients at two city hospitals to evacuate.

A Hilton Hotel was smouldering and in ruins.

"Imagine a wind-whipped fire burning at explosive rates. This is 50 miles per hour. Literally, it's burning into the city of Santa Rosa ... burning box stores," Pimlott said.

"This is traditionally California's worst time for fires, California's most damaging times for fires have occurred in October."

More than 200 people were hurriedly evacuated from Sutter Santa Rosa Regional Hospital and Santa Rosa Medical Center, where the blaze could be seen raging nearby.

State of emergency

Barry Biermann, Napa county fire chief, said the fires had burned more than 176sq km.

Crews had not yet been able to contain a fire heading towards Napa's business district.

"Right now, with these conditions, we can't get ahead of this fire and do anything about the forward progress," Biermann said.

California Governor Jerry Brown declared a state of emergency in Napa, Sonoma and Yuba counties.

Smoke was thick in San Francisco, 96km south of the Sonoma County fire.

The National Weather Service said widespread wind gusts were observed in the north San Francisco Bay region.

The winds were expected to subside at midday.

Community centres, the Sonoma County Fairgrounds and other local centres were opened for evacuees.