The order exempts businesses and agencies that provide essential services, like grocery stores, pharmacies, gas stations and doctor’s offices, or are considered critical infrastructure.

It has no specific end date.

[Watch Mr. Newsom’s full news conference on the new order here.]

The move added a level of consistency to restrictions that have been enacted piecemeal over the last several days across the state.

In large swaths of California, “nonessential” businesses like movie theaters, gyms and bars had already been ordered to close. Restaurants had been limited to takeout or delivery only. And state officials had already urged residents 65 and older or with chronic health conditions to stay at home.

Mr. Newsom’s announcement came two days after public health officials in seven counties around the Bay Area ordered their roughly 7 million residents to shelter in place, setting in motion a vast, unprecedented experiment.

[What does it mean to “shelter in place”? Here are answers to your questions.]

Other communities followed, adopting almost identical orders or measures with slight variations.