NEW YORK — With masks, ventilators and political goodwill in desperately short supply, more than one-fifth of the world’s population was ordered or urged to stay in their homes Monday at the start of what could be a pivotal week in the battle to contain the coronavirus in the U.S. and Europe.

Partisan divisions stalled efforts to pass a colossal aid package in Congress, and stocks fell again on Wall Street even after the Federal Reserve said it will lend to small and large businesses and local governments to help them through the crisis.

Warning that the outbreak is accelerating, the head of the World Health Organization called on countries to take strong, coordinated action.

“We are not helpless bystanders,” Tedros Adhanom Ghebreyesus said, noting that it took 67 days to reach 100,000 cases worldwide but just four days to go from 200,000 to 300,000. “We can change the trajectory of this pandemic.”

Britain became the latest European country to order a near lockdown, imposing its most draconian restrictions ever in peacetime.

The scramble to marshal public health and political resources intensified in New York, where a statewide lockdown took effect amid worries the city of 8.4 million is becoming one of the world’s biggest hot spots. More than 12,000 people have tested positive in the city and almost 100 have died.

The governor announced plans to convert a mammoth New York City convention center into a hospital with 1,000 beds. The mayor warned that the city’s hospitals are just 10 days away from shortages in basic supplies.

“This is going to get much worse before it gets better. We are still in the relative calm before the storm,” Gov. Andrew Cuomo said at the Jacob K. Javits Convention Center.

In Italy, the hardest-hit country, declines in both new cases and deaths for a second consecutive day provided a faint glimmer of hope, though it is too soon to say whether the crisis is leveling off.

Italian officials said Monday that the virus had claimed just over 600 more lives, down from 793 two days earlier. All told, the outbreak has killed more than 6,000 Italians, the highest death toll of any country, and pushed the health system to the breaking point there and in Spain.

The risk to doctors, nurses and others on the front lines has become plain: Italy has seen at least 18 doctors with coronavirus die. Spain reported that more than 3,900 health care workers have become infected, accounting for roughly 12% of the country’s total cases.

British health workers pleaded for more gear, saying they felt like “cannon fodder.” In France, doctors scrounged masks from construction workers, factory floors, an architect.

“There’s a wild race to get surgical masks,” Francois Blanchecott, a biologist on the front lines of testing, told France Inter radio. “We’re asking mayors’ offices, industries, any enterprises that might have a store of masks.”

The way U.S. officials respond to the severe pressure on hospitals — and people’s willingness to keep their distance from others — will prove critical in coming days, public health experts said.

“Actions taken right now will have a huge impact on the course of this epidemic in the U.S.,” said Josh Michaud, associate director of global health policy with the Kaiser Family Foundation in Washington. “It’s an important moment.”

British Prime Minister Boris Johnson on Monday directed residents to stay home, with a few exceptions, and ordered shops that don’t sell essential goods to shut down.