Tens of thousands of people have been ordered to evacuate after a fast-moving wildfire exploded in size.

The blaze threatened several northern California communities and forced panicked residents to race to help neighbours who had to drive through walls of flames to escape.

Butte County Sheriff’s spokeswoman Miranda Bowersox said that all of Paradise, a town of about 27,000 people 180 miles north east of San Francisco, was ordered to evacuate.

“It’s bad,” Butte County Sheriff Kory Honea told the Enterprise-Record. “We’re trying to get as many people out as quickly as possible and save as many lives as we can.”

He confirmed reports that evacuees had to abandon their vehicles as they fled the scene.

“We’re getting them on other vehicles with room. We’re working very hard to get people out. The message I want to get out is if you can evacuate, you need to evacuate,” he said.

Rick Carhart, a Cal Fire spokesman, said the wildfire was reported at 6.30am, affecting about 30,000 people in the towns of Paradise, Concow and other small communities.

“The blaze is being driven by fairly strong winds,” Mr Carhart said. “It’s really dry and we have low humidity and unfortunately those are great conditions for a fire to spread.”

Thick grey smoke and ash filled the sky above Paradise and could be seen from miles away.

The Adventist Health Feather River Hospital in Paradise evacuated all of its patients and staff, given its close proximity to the fire, and transported them safely to hospitals in neighbouring towns.

Four of the hospital’s employees were briefly trapped in the basement and rescued by California Highway Patrol officers.

Shari Bernacett said she and her husband tried to get people to leave the mobile home park they manage in Paradise and had minutes to evacuate as flames reached the east side of the town.

Ms Bernacett said she and her husband “knocked on doors, yelled and screamed” to alert as many of the residents of 53 mobile homes and recreational vehicles as possible to leave.

“My husband tried his best to get everybody out. The whole hill’s on fire. God help us!” Ms Bernacett said before breaking down crying.

She and her husband grabbed their dog, jumped in their pick-up truck and drove through flames before getting to safety on Highway 99, she said.

Mr Carhart said officials were sending as many crews as they could gather.

“Every engine that we could put on the fire, is on the fire right now and more are coming,” he said. “There are dozens of strike teams that we’re bringing in from all parts of the state.”

The National Weather Service issued red flag warnings for fire dangers in many areas of the state, saying low humidity and strong winds were expected to continue through Friday evening.