California’s 40 million residents have been ordered to stay at home as the coronavirus pandemic worsens across the United States.

State governor Gavin Newsom made the unprecedented order on Friday (Australian time), just hours after Los Angeles mayor Eric Garcetti ordered residents of the US’s second largest city into lockdown and for “non-essential” businesses to shut.

Effective immediately, Mr Newsom’s order is the largest and most sweeping government clampdown yet in the worsening public health crisis brought on by the COVID-19 outbreak.

He said modelling had shown that 56 per cent of Californians – more than 25 million people – were expected to contract COVID-19 in the next eight weeks, requiring nearly 20,000 more hospital beds than the state can provide.

California, which is among the hardest hit US states in the COVID-19 outbreak, already has more than 1000 confirmed cases and nearly 20 deaths.

“We are confident the people of California will abide by it, they will meet this moment,” Mr Newsom said.

“They’ll step up as they have over the last number of weeks to protect themselves, to protect their families and to protect the broader community in this great state and the world we reside in.”

Mr Newsom said exceptions to the stay-at-home ordered would be granted for residents to make necessary trips to supermarkets, pharmacies, doctors and, for some, work. The order has no end date, but he has suggested it will last at least eight weeks.

He had also asked US President Donald Trump to send a US Navy hospital ship to Los Angeles immediately as California braces for the expected surge in coronavirus cases.

A spokesman for Mr Newsom, Nathan Click, said the governor had intended to make the threat of the virus clear to Californians.

“Governor Newsom has been honest about the threat of the virus and its impact on the health and welfare of Californians,” he told the Los Angeles Times.

“This projection shows why it’s so critical that Californians take action to slow the spread of the disease – and those mitigation efforts aren’t taken into account in those numbers.

“The state is deploying every resource at its disposal to meet this challenge, and we continue to ask for the federal government’s assistance in this fight.”

In a separate letter to US congressional leaders, Mr Newsom also asked for additional federal funding for unemployment insurance and social safety net programs and aid to small businesses, schools and universities.

Disneyland at Anaheim has already closed due to the pandemic. Photo: Getty

Earlier, LA mayor Eric Garcetti said the lockdown orders would mean all indoor shopping malls, playgrounds and other gathering places would be shut at least until the end of March.

“We’re taking this urgent action to limit the spread of COVID-19 and save lives,” he said.

Similar so-called “shelter in place” directives have also been given in more than a dozen northern California counties.

-with AAP