LOS ANGELES — California on Thursday imposed a curfew across much of the state to stem the rapid rise of coronavirus cases.

Governor Gavin Newsom said the 10:00 pm to 5:00 am stay-at-home order will take effect Saturday evening and remain in force through December 21.

“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,” he said in a statement.

“It is crucial that we act to decrease transmission and slow hospitalizations before the death count surges.

“We’ve done it before and we must do it again.”

The order will be effective in counties in the state’s most restrictive purple tier, which applies to 94 percent of the state’s population including in Los Angeles and San Diego.

San Francisco, which moved from the least-restrictive yellow to red tier this week, will not be affected by the curfew.

However the city, like much of the Bay Area, has been forced to shut down indoor dining and has restricted retailers and gyms amid a renewed coronavirus surge.

Authorities said COVID-19 cases have increased by approximately 50 percent in California during the first week of November, sparking fears of hospitals in hot-spots being overwhelmed with new patients.

“Letting our guard down could put thousands of lives in danger and cripple our health care system,” said Erica Pan, the state’s acting public health officer.

“Together we prevented a public health crisis in the spring and together we can do it again,” she added.

Mark Ghaly, the state’s health and human services secretary, warned that harsher restrictions could be imposed unless the number of cases goes down.

“We may need to take more stringent actions if we are unable to flatten the curve quickly,” he said. “Taking these hard, temporary actions now could help prevent future shutdowns.”

Newsom’s announcement on the curfew came as more and more US states rolled back on reopenings.

America has now registered over 251,000 fatalities and over 11.6 million reported cases, according to a running tally by Johns Hopkins University, by far the highest reported national death toll.

The surge in cases has alarmed authorities to the point that the US Centers for Disease Control and Prevention urged Americans not to travel for next week’s Thanksgiving holiday.