Gov. Gavin Newsom speaks at a coronavirus briefing in Sacramento in July.

(AP) — California is imposing an overnight curfew on most residents as the most populous state tries to head off a surge in coronavirus cases that it fears could tax the state’s health care system, Gov. Gavin Newsom announced Thursday.

What officials are calling a limited stay-at-home order requires nonessential residents to stay home from 10 p.m. to 5 a.m. starting Saturday.

Fresno Mayor Lee Brand asked in a statement sent to the press that Fresno residents “voluntarily comply” with the order. Police would focus, however, on their regular responsibilities.

“In light of the steep surge in positive cases we are currently experiencing, and as with other COVID-19 orders designed to protect the most vulnerable members of our community, the City of Fresno is asking our residents to voluntarily comply with the State’s limited stay-at-home directive. Our police department will concentrate on their regular responsibilities to serve and protect the people of Fresno,” Brand said.

It affects counties with the most severe restrictions, 41 of the state’s 58 counties that are in the “purple” tier under California’s color-coded system for reopening the economy. That covers 94% of the state’s nearly 40 million residents.

The move comes only days after the state imposed restrictions limiting business operations in those 41 counties, which have the most significant increases in virus cases.

“The virus is spreading at a pace we haven’t seen since the start of this pandemic and the next several days and weeks will be critical to stop the surge. We are sounding the alarm,” Newsom said in a statement.

The order will last one month, until Dec. 21, but could be extended if infection rates and disease trends don’t improve.

While nonessential businesses must close by 10 p.m., restaurants will be permitted to offer takeout food and people can perform some routine activities like walking the dog, officials said. They will still be able to get medical care, pick up prescriptions and take care of other essential needs.

Officials said overnight movements are more likely to involve social activities that bring increased risk of infection, particularly if people drink and let down their guard on basic safety precautions like wearing masks and staying a safe distance apart.

It follows the state’s more sweeping lockdown in the spring that affected all residents, day and night.

“We know from our stay at home order this spring, which flattened the curve in California, that reducing the movement and mixing of individuals dramatically decreases COVID-19 spread, hospitalizations, and deaths,” California Health and Human Services Secretary Dr. Mark Ghaly said in a statement. “We may need to take more stringent actions if we are unable to flatten the curve quickly. Taking these hard, temporary actions now could help prevent future shutdowns.”