Wildfires fueled by strong and unpredictable winds raged across southern California on Thursday, sweeping into the upscale suburbs of Los Angeles, America's second-largest city.

Tens of thousands of people fled their homes amid apocalyptic scenes of entire hillsides engulfed in billowing smoke and towering plumes of flame.

Multi-million dollar mansions were destroyed in the Bel-Air neighborhood, where celebrities own homes and media tycoon Rupert Murdoch has a vineyard.

The University of California, Los Angeles (UCLA) cancelled classes as firefighters battled the "Skirball fire" west of the sprawling campus.

Los Angeles city officials issued a countywide emergency alert warning that the seasonal Santa Ana winds were creating an "extreme fire danger."

Further to the north in Ventura County, an even bigger blaze, the "Thomas fire," was raging and threatening several coastal and inland towns.

Major north-south highways, including the famed Route 101, were closed as the fire jumped over the road and menaced beachfront homes.

The Thomas fire has consumed 96,000 acres (38,850 hectares) and forced 50,000 people to flee their homes, the California Department of Forestry and Fire Protection (Cal Fire) said.

Cal Fire chief Ken Pimlott said the strong winds, including gusts of up to 80 miles per hour (130 kilometers per hour) were hampering efforts to combat the blazes.

"There will be no ability to fight fire in these kinds of winds," Pimlott said.

Celebrities flee Bel-Air mansions 

Los Angeles Mayor Eric Garcetti said more than 230,000 people had been forced from their homes in Los Angeles and Ventura counties.

The "Skirball" fire ignited Wednesday morning and began to sweep through affluent Bel-Air, home to celebrities and billionaires including SpaceX CEO Elon Musk and pop superstar Beyonce.

Police knocked on doors and used loudspeakers to make sure everybody had left their homes.

The singer Lionel Richie cancelled a concert to help his ex-wife flee the area, while comedian Chelsea Handler and designer Adrienne Maloof were among those tweeting that they had to evacuate.

Among those evacuated was model Chrissy Teigen, wife of singer John Legend.

"Never thought I'd get to actually play what I thought was a hypothetical game of what would you grab if there were a fire," Teigen wrote on Twitter.

"We are fine and we will be fine. Thinking of everyone else affected and continuing my lifelong intense love of firefighters," she added.

Forecasters predicted that winds could cause fires to spread further, threatening more homes and the acclaimed Getty Center museum.

The Getty -- home to masterpieces including works by Edouard Manet -- was closed. Museum authorities tweeted that "air filtration systems are protecting the galleries from smoke."

Residents of wealthy Los Angeles neighborhoods between Mulholland Drive to the north and Sunset Boulevard to the south were part of the evacuation zone.

At least 4,000 firefighters have been deployed to fight the fires and Governor Jerry Brown has declared a state of emergency.

The Federal Emergency Management Agency (FEMA) said it had released funds for relief services.

President Donald Trump tweeted a message of "thoughts and prayers" to California as it nears the end of its deadliest year ever for wildfires.