LOS ANGELES — America’s most populous state is ordering its residents to stay indoors.

Gov. Gavin Newsom of California on Thursday ordered Californians — all 40 million of them — to stay in their houses as much as possible in the coming weeks as the state confronts the escalating coronavirus outbreak. The order represents the most drastic measure any governor has taken to control the virus, and a decision that Gov. Andrew M. Cuomo of New York, which has far more cases than California, has resisted taking.

Newsom made the announcement from the state’s emergency operations center in Sacramento, normally a place where emergency workers coordinate responses to wildfires and earthquakes, and spoke in stark terms of the risk that the coronavirus posed to the population.

Citing a model that state planners have been using, suggesting that 56% of Californians, or more than 25 million people, could be infected over eight weeks, Newsom said, “I think it’s time I tell you what I tell my family.”

“This is not a permanent state, this is a moment in time,” he said. “We will look back at these decisions as pivotal.”

Newsom said that most retail shops, including indoor malls, are being ordered shut across the state. Also closed are most corporate offices. Banks, grocery stores, pharmacies, laundromats and some other businesses are exempted.

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Officials emphasized that the orders did not bar residents from leaving their homes, and they encouraged people to take walks, as long as they stay 6 feet apart, and visit grocery stores.

Health care workers, essential municipal workers such as bus drivers and others will still be working.

Earlier in the week several counties in the Bay Area, along with Sacramento, issued orders that residents essentially shelter in place, although there are several exceptions — which also apply to the state order.

The new rules were the most drastic ones so far in the country for the population size covered, and follow similar crackdowns in Europe, most notably in Italy, where the death toll from the relentless virus Thursday surpassed that of China.

Just before Newsom spoke, officials in Los Angeles County held a news conference to announce their own stay-at-home order, which they are calling “safer at home.”

How the orders will be enforced is unclear, but officials said they expected residents to follow them and that there would enormous social pressure to do so on those who disobey them.

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“People will self-regulate their behavior,” he said. “We’ll have social pressure to encourage people to do the right thing.”

A Los Angeles County sheriff’s official said on Twitter that the department did not plan on making arrests to enforce the order.

Newsom did, however, say that the state would be “more aggressively” policing xenophobic attacks against Asians. “We are better than that,” Newsom said.

Thursday’s directives in Los Angeles and from the governor come after several days in which California leaders, both at the state and local level, gradually tightened public life, closing bars, wineries, gyms and movie theaters and ordering restaurants to halt in-housing dining and shift to takeout and delivery. On Sunday night Mayor Eric Garcetti of Los Angeles announced that all bars would close and restaurants would be allowed only to do takeout or delivery. In recent days he has also been urging people to stay home from work.

Officials have described the stay at home orders as an aggressive way to reduce infections and buy time to stock up on medical supplies like ventilators and masks to confront what they expect to be a surge in need for hospital beds.

Also Thursday, Newsom released a letter he had sent to President Donald Trump asking him to deploy a hospital ship to the Port of Los Angeles to be on standby for an expected surge in hospitalizations because of the rapidly spreading coronavirus outbreak. In making his plea, Newsom cited the alarming statistic that roughly 56% of the state’s population, or 25.5 million people — in line with some national projections about how severe the outbreak could become — “will be infected with the virus over an eight-week period.

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“In some parts of our state, our case rate is doubling every four days,” he wrote.

But Thursday night, he said he hoped that the extraordinary measures would stop those projections from becoming real.

“The point of the stay at home order is to make those numbers moot,” Newsom said.

California’s Department of Public Health on Thursday reported the state had 675 confirmed cases and 16 deaths, up from 598 cases and 13 deaths the day before. One of the new deaths came in Los Angeles County, where officials said the region had 231 cases — they reported 40 new cases Thursday. The most recent death, the county’s public health department said, was a 30-something who had underlying conditions and lived near Pasadena.