California's 40 million residents have been told to only leave their homes when absolutely necessary during the coronavirus pandemic.
State governor Gavin Newsom issued the state-wide "stay at home" order, which has taken effect from Thursday evening and will remain in place until further notice.
He said the state needed to "recognise the reality" and "make tough decisions", although conceded that home isolation was not his "preferred choice".
The governor said modelling has shown 56% of California residents are expected to contract COVID-19 over the next eight weeks - requiring nearly 20,000 more hospital beds than the state can currently provide.
He warned infection rates are doubling every four days in some parts of the state.
CA is issuing a statewide, mandatory STAY AT HOME order.
Those that work in critical sectors should go to work. Grocery stores, pharmacies, banks and more will stay open.
We need to meet this moment and flatten the curve together.
Go to https://t.co/xtXFwVeWc2 to learn more.
— Gavin Newsom (@GavinNewsom) March 20, 2020
Los Angeles, the second-largest US city, is likely to be "disproportionately impacted" in the coming weeks, Mr Newsom added.
The new order means non-essential services including restaurants, bars and gyms will shut down.
However the home isolation measures will not be enforced by police.
Earlier on Thursday, Mr Newsom asked US President Donald Trump to send a US Navy hospital ship to the port of Los Angeles "immediately" to deal with an expected surge in coronavirus cases.
The US State Department has issued a travel alert, urging Americans not to go abroad under any circumstances and to return home if they are already abroad unless they plan to remain overseas.
The US has also been considering measures to immediately turn back anyone who crosses the border from Mexico illegally, including asylum seekers.
Elsewhere, Iran's mission to the United Nations has said the "inhumane and unlawful" US sanctions are hampering efforts to prevent COVID-19 cases in the country, urging the international community to help lift the sanctions.
Haiti has confirmed its first two cases of the virus and announced it will close all airports, schools, factories and seaports.
The Cannes Film Festival in France has been postponed, with organisers saying they are considering moving the festival to June or July.
And South Korea will test all passengers coming from Europe for coronavirus and enforce two-week quarantines beginning on Sunday.
Australia has demanded indoor venues provide at least four square metres of space between each person and tightened regulations on travel to and from remote indigenous communities.