Paradise – population 27,000 – is said to have been left in ruins with homes, hospitals, schools and businesses all engulfed by a wall of flames.
Authorities said there are unconfirmed reports of casualties and that emergency services are working desperately to rescue those left behind.
“Pretty much the community of Paradise is destroyed,” Cal Fire Captain Scott McLean told the Associated Press news agency. “It's that kind of devastation. The wind that was predicted came and just wiped it out.”
A police officer who lives in the city, in Butte County, added: “It was just a wall of fire on each side of us. We could hardly see the road in front of us.”
The devastation is only the worst so far during two days of horror as fires, fanned by 50 mph winds, raged, split up, and spread across vast swathes of the state. The smoke is so thick it is visible from space.
Several more towns and communities – including Magalia, Concow, and Butte Creek Canyon – have been entirely evacuated after acting California governor Gavin Newsom declared a state of emergency.
The largest of the blazes, Camp Fire, in the state’s north, has scorched over 20,000 acres, officials say. Homes, hospitals, nursing homes, restaurants and schools are among buildings said to have been destroyed. Thousands more have been left without power.
In Southern California, meanwhile, the Hill Fire has grown to 10,000 acres and late on Thursday jumped a major highway which authorities had hoped would act as a firebreak.
Among nearby residents who have been evacuated are Kim Kardashian, the Evening Standard reports.
Smoke could also be seen hundreds of miles away in Thousand Oaks, the scene of the mass bar shooting that left 12 people dead on Thursday.
Strong winds, low humidity and dry forests have all exacerbated the problem. By Friday morning, the largest three fires were all still at “zero per cent containment", officials said.
The region has grappled with serious and persistent wildfires - including the worst in the state's history, the Mendocino and Thomas fires - on and off since 2017.