Southern California went into a strict lockdown Monday, with more than 20 million people under stay-at-home orders triggered after hospitals faced being overwhelmed by record Covid-19 cases.
Governor Gavin Newsom last week announced new statewide bans on gatherings and "non-essential" activities would come into effect once 85 percent of intensive care unit beds have been filled.
Under the lockdown, most offices will close and gatherings of people from different households are banned. Bars and personal services such as hair salons will be temporarily closed, and restaurants will only be allowed to offer take-out and delivery.
"We are at a tipping point in our fight against the virus and we need to take decisive action now to prevent California's hospital system from being overwhelmed in the coming weeks," Newsom said.
By implementing stay-at-home orders when ICU capacity falls below 15 percent, "we can flatten the curve as we've done before and reduce stress on our health care system."
California's Department of Public Health said on Sunday that ICU capacity in Southern California and San Joaquin Valley had fallen below 15 percent.
"The regional stay at home order will take effect in those two regions tonight at 11:59 pm and will remain in effect for at least three weeks," it said.
California is experiencing record new cases of Covid-19 in a third wave of the virus, with two days last week in which 113 deaths were recorded, up from around 14 daily fatalities in early November.
Non-essential travel is also temporarily restricted statewide and hotels will no longer be allowed to cater to tourists.
Offices must close, except for critical infrastructure sectors where remote working is not possible.
Shops will be limited at 20 percent capacity, while only schools with waivers will be allowed to remain open.
California, the most populous US state, has seen more than 1.3 million cases and nearly 20,000 deaths. It recorded 30,075 new infections on Saturday.
Newsom said there would be new grants and tax relief to help small business owners who have been hard-hit.
The measures follow similar bans on gatherings that went into place this week in densely populated and badly hit Los Angeles County.
They extend upon a near-statewide nighttime curfew introduced last month.
But the new, stricter measures still fall short of California's first lockdown in March, which saw most outdoor activities including beach access and hiking banned.
In northern California, San Francisco has also implemented a stay at home order, which went into effect Sunday night due to a sharp increase in Covid-19 cases and hospitalizations.
"Within weeks we may not be able to care for people at our hospitals if we don't take action NOW," the San Francisco Department of Emergency Management tweeted as it announced the lockdown.