A storm of historic proportions unleashed record levels of rain over parts of Los Angeles on Monday, sending mud and boulders down hillsides dotted with multimillion-pound homes, posing a grave danger to the city’s large homeless population and knocking out power for more than a million people in California.

The storm was the second one fuelled by an atmospheric river to hit the state in just a few days.

About 1.4 million people in the Los Angeles area, including the Hollywood Hills and Beverly Hills, were under a flash flood warning on Monday. Up to nine inches of rain had already fallen in the area, with more expected, according to the National Weather Service, which called the flash flooding and threat of mudslides “a particularly dangerous situation”.

Already crews were rescuing people from swift-moving water in various parts of southern California.

Gushing rivers carried mud, rocks and objects from people’s multimillion-pound homes, including coolers, ladders and plastic crates, in Studio City, at the back of the Hollywood Hills.

Several homes were damaged, including one with a crumpled garage door from the debris slide.

A record 4.1 inches of rain fell on Sunday in central Los Angeles, beating the previous record of 2.55 inches set in 1927, the National Weather Service said.

The weather service forecast up to eight inches of rain across southern California’s coastal and valley areas, with 14 inches possible in the foothills and mountains over the next two days.

More than 474,061 homes, businesses or other locations were without electricity statewide on Monday, according to poweroutage.us.

Commuters stepped through several inches of floodwater as they went to catch trains at Union Station in central Los Angeles.

Governor Gavin Newsom declared a state of emergency for Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura counties.

The Governor’s office of emergency services activated its operations centre and positioned personnel and equipment in areas most at risk.

Evacuation orders were issued in many parts, stretching from Santa Barbara to Los Angeles Counties where emergency shelters were in place.