California officials on Monday pulled what they characterized as an emergency brake, announcing a dramatic rollback of reopening in much of the state as it broke its single-day record for new coronavirus cases.

Once the changes go into effect Tuesday, 94% of Californians — roughly 37 million people — will live in counties that are in the strictest tier of the state’s four-tier, color-coded reopening road map. Many businesses in those counties will have to suspend or severely limit their indoor operations.

The move came as California recorded more than 13,280 new cases Monday night, according to The Times’ coronavirus tracker. That is the highest daily total since the pandemic began and a sign of how quickly COVID-19 is spreading.

“We are seeing community spread broadly now throughout the state of California,” Gov. Gavin Newsom said.

Last week, just 13 of California’s 58 counties were in the strictest tier, purple, indicating widespread transmission of the disease. Newsom announced Monday that the number would more than triple, to 41 counties.

Among the counties that have newly fallen back into the purple tier are Orange, Ventura, Santa Barbara, Kern and San Luis Obispo. Los Angeles, San Bernardino, Riverside, San Diego and Imperial counties were already in that category.

The most populated counties in the Bay Area were also sent back to the purple tier, including Santa Clara, Alameda, Contra Costa, Napa and Solano. All eight counties in the San Joaquin Valley are now in that tier; the purple swath also covers all of the Central Coast and much of the area in and around Sacramento County.

Monday’s announcement came amid the steepest rise in coronavirus cases that California has seen. Over the seven-day period that ended Sunday, the state averaged 7,985 cases per day, a rate that has nearly doubled in the last two weeks, according to data compiled by The Times.

The rolling metric hasn’t been that high since mid-August and is a far cry from just a month ago, when the seven-day average of new cases dipped below 3,000.

“This is simply the fastest increase California has seen since the beginning of this pandemic,” Newsom said.

Los Angeles County reported more than 3,000 new infections on both Saturday and Sunday; the 3,780 cases on Saturday represented the highest one-day total since mid-July. Even on Monday, when case counts are typically lower because of reporting lags over the weekend, officials announced 2,795 new cases.

“It is clear that L.A. County is at a very dangerous point in the pandemic,” county Public Health Director Barbara Ferrer said.

As the case count swells, officials stress that it’s essential for residents to follow infection prevention protocols: Wear a mask in public, wash hands regularly, stay home when sick and keep a physical distance from, and avoiding gathering with, people from outside your household.

None of those are novel concepts in the battle against the coronavirus. As Ferrer put it, “This isn’t like we don’t know what to do. This is about getting back to doing what works.”

“As a community and as individuals, we’re going to need to be laser-focused on reducing transmission,” she said during a briefing. “To say many lives are at stake is not an exaggeration.”

The widespread reassignment announced Monday means that only 17 counties won’t be subject to the most stringent restrictions on businesses and other facilities outlined in California’s reopening strategy.

Once the changes go into effect, 11 counties will be in the second-most severe, or red, category: Del Norte, Modoc, Humboldt, Plumas, Colusa, Lake, Marin, San Francisco, San Mateo, Amador and Mono.

Four are in the third tier, orange — Lassen, Sierra, Calaveras and Inyo — and only two, Alpine and Mariposa, remain in the least restrictive yellow tier.

Even more changes are coming. California has been updating its tier assignments weekly, on Tuesdays, but Newsom said those determinations will now be made and announced closer to real time.

“We need now to be more aggressive,” he said, “more surgical, more targeted.”

The tier system categorizes counties based on their average daily number of new coronavirus cases for every 100,000 residents and the percentage of conducted tests that detect the virus.

Before, counties would regress only if their metrics fell in the range of a more restrictive tier for two straight weeks. Now, those decisions can be made based on one week’s worth of data.

Some counties saw their metrics worsen this week to the point that they slid back multiple categories. Among them was San Francisco, which was long hailed as a coronavirus success story but is now “seeing an explosion of new cases throughout the city,” Public Health Director Dr. Grant Colfax said in a news conference.

Though some of California’s increased case count is likely a byproduct of ramped-up testing, the rate at which tests are coming back positive is also increasing. About a month ago, the seven-day positivity rate was 2.58%; it is now 5%.

The number of those infected who are ill enough to require hospitalization has also surged in recent weeks. As of the latest tally, there were 3,852 COVID-19 hospitalizations statewide — up roughly 48% from two weeks ago. There are 1,031 COVID-19 patients in intensive care.

“As I say to my patients often, ‘When I’m worried, I’ll tell you, so you can worry with me.’ And we’re there,” said Dr. Mark Ghaly, California’s Health and Human Services secretary.

Eventually, “about 12% of those cases end up in the hospital,” he said during a briefing Friday, “and when you look at a high number of cases day over day, and that’s sustained, that’s exactly what we worry about in terms of overwhelming our hospital system.”

More than 1 million Californians have been diagnosed with COVID-19 at some point during the pandemic. Although most have recovered, more than 18,200 have died.

The spate of new cases comes as California — and the rest of the nation — heads into what officials already predict will be a particularly dangerous time for the pandemic, when lower temperatures push people indoors and holidays tempt them to gather.

Last week, Newsom issued an advisory recommending that California residents avoid unnecessary travel — including for Thanksgiving — and urging those who do head out of the state to self-quarantine for 14 days when they return.

As the latest surge shows no signs of slowing, some Los Angeles County officials have discussed potentially limiting businesses’ hours or instituting some type of curfew in a bid to stymie community spread.

Newsom said the state also was assessing the possibility of a curfew.

Though health officials have regularly preached that personal responsibility is key to warding off the worst the pandemic can bring, it remains to be seen what additional measures California could contemplate if that message falls on deaf ears.

That’s an especially worrying possibility as some residents and businesses grow increasingly tired of, or resentful toward, the sacrifices they have been asked to make for months.

Among those who have stumbled is Newsom, who came under fire after attending a birthday party with members of other households at an upscale Napa Valley restaurant.

“I want to apologize to you, because I need to preach and practice, not just preach and not practice, and I’ve done my best to do that,” he said Monday. “We’re all human. We all fall short sometimes.”

Thanksgiving could prove a particularly hard sell for pandemic-weary Californians. The state has generally banned large gatherings — although brief, small ones of no more than three households may be held, provided they take place outdoors and attendees physically distance and wear face coverings.

Whether large numbers of families will willingly forgo fellowship and food around the Thanksgiving table remains to be seen.

Officials and experts say it’s safest to connect with extended friends and family through video calls or by phone, while limiting in-person celebrations to those you live with.

“I know everybody wants to know what to do for Thanksgiving, and the basic answer — to be the Grinch who stole Thanksgiving — is not to do it,” Dr. George Rutherford, an epidemiologist and infectious disease expert at UC San Francisco, said at a recent campus town hall meeting.

Times staff writers Maura Dolan, Taryn Luna, Alex Wigglesworth, David Zahniser and Iris Lee contributed to this report.