In another extraordinary move, Mr. Mnuchin said Mr. Trump instructed him to allow for the deferment of income tax payments, interest free and penalty free, for 90 days. People can defer up to $1 million and corporations can defer up to $10 million.

The Treasury secretary said that would temporarily inject $300 billion into the economy, cushioning the blow of lost wages and customers.

The aggressive series of actions came on a day when much of the country seemed to grind to a halt, with daily rituals like commuting and dining out replaced by hunkering down at home. The death toll from the novel coronavirus rose to 100 in the United States, with 5,600 cases confirmed and, with the first case reported in West Virginia, the disease present in all 50 states.

In New York City, Mayor Bill de Blasio warned residents to prepare for the possibility of a “shelter in place” order within the next 48 hours. Officials in North Carolina’s Outer Banks set up checkpoints to turn away visitors. Beach groups of more than 10 were banned in Florida after photographs showed packed Gulf Coast beaches this week during spring break.

To help ease the growing strain of the virus on the United States’ medical system, the Pentagon said it was making available 2,000 ventilators for hospitals, a number that would most likely fall far short of the expected need.

Urgent measures that until recently would have been unthinkable are also being put in place around the world. In the European Union, leaders voted on Tuesday to close off at least 26 countries to nearly all visitors from the rest of the world for at least 30 days

After the briefing at the White House by Mr. Trump and his coronavirus task force, Mr. Mnuchin went to Capitol Hill. There, he told Republican senators at their weekly lunch that the Trump administration wanted Congress to infuse about $1 trillion in additional stimulus to prop up the economy. While Mr. Trump had previously signaled that he wanted the payroll tax to be the centerpiece of that effort, it faced bipartisan opposition in Congress, and Mr. Mnuchin said time was of the essence.