By Peter Baker, New York Times–

President Trump on Wednesday night blocked most visitors from continental Europe to the United States and vowed emergency aid to workers and small businesses as the World Health Organization declared the coronavirus a global pandemic, stock markets plunged further and millions of people cut themselves off from their regular lives.

In a prime-time address from the Oval Office, Mr. Trump outlined a series of measures intended to tackle the virus and its economic impact as he sought to reassure Americans that he was taking the crisis seriously after previously playing down the scope of the outbreak. He said he would halt travelers from Europe other than Britain for 30 days and asked Congress to support measures like a payroll tax cut.

“The virus will not have a chance against us,” Mr. Trump declared in his 10-minute speech, reading from a teleprompter in an uncharacteristic monotone. After weeks of quarreling with rivals over his response, even calling their criticisms a “hoax,” he called on them to stand down. “We are all in this together,” he said. “We must put politics aside, stop the partisanship and unify together as one nation and one family.”

The president’s address came as the virus sent stock markets deeper into a meltdown, prompted the N.C.A.A. to bar crowds from its iconic March Madness annual basketball tournament and forced the N.B.A. to suspend its season altogether after one of its players tested positive. Schools, universities, businesses, theaters and sports stadiums shut their doors. And the actor Tom Hanks announced that he and his wife, Rita Wilson, had been infected with the virus.

The cascade of announcements felt like a turning point in the crisis, when the real-world effect on people in the United States and around the globe came into stark relief. Ordinary life in many places will no longer be the same for the foreseeable future as society adjusts to a new reality that transforms everything including the global economy and everyday social interactions — not just in far-off places on newscasts, but in the community right at home.

Mr. Trump and other world leaders grappled for a way forward, but there was no clear end in sight as one of America’s top scientists, Dr. Anthony S. Fauci, predicted the outbreak would only grow worse. He spoke on the same day that India joined countries like China, Italy, Iran, Japan and Israel in imposing drastic travel limits.

Italy went further by ordering almost all nonessential businesses to close, including restaurants, bars, cafes, beauty salons and most stores. In Germany, Chancellor Angela Merkel warned that about two-thirds of her country’s population may eventually be infected, a prediction that rattled many in Europe and across the ocean.

With the virus now in more than 100 countries, the W.H.O. cited the “alarming levels of inaction” in declaring a global pandemic, the first time it has used that designation since 2009. Although the term is largely symbolic, the decision to use it was a sign of growing concern among public health officials about the failure to contain the virus, which has infected more than 120,000 people and killed more than 4,300 around the world.

“Pandemic is not a word to use lightly or carelessly,” Dr. Tedros Adhanom Ghebreyesus, the head of the W.H.O., said at a news conference in Geneva. “We cannot say this loudly enough or clearly enough or often enough,” he added. “All countries can still change the course of this pandemic.”

As of late Wednesday, at least 1,240 people in 42 states and Washington, D.C., had tested positive for the coronavirus, according to a New York Times database, and at least 37 patients with the virus had died.

In New York, as in other world financial centers, already shaky markets fell another 5 percent to 6 percent, officially putting them into bear market territory for the first time in 11 years. In Washington, Dr. Fauci, the director of the National Institute of Allergy and Infectious Diseases, warned that the escalating outbreak should prompt organizations to scrap large gatherings.

“The bottom line: It is going to get worse,” he said at a congressional hearing. “We would recommend that there not be large crowds,” he added, citing as an example N.B.A. games.

Hours later, the N.B.A. announced that a member of the Utah Jazz tested positive for the coronavirus shortly before the tipoff of Wednesday night’s game against the Oklahoma City Thunder. The game was promptly canceled and the N.B.A. called off the rest of its games “until further notice,” adding, “The N.B.A. will use this hiatus to determine next steps for moving forward in regard to the coronavirus pandemic.”

Mr. Trump and other world leaders struggled to get ahead of the situation without a clear path forward. A day after he called for calm and assured Americans that the virus “will go away,” Mr. Trump signaled that the crisis was worse than he was saying by deciding to address the nation. But even then, he sought to minimize it as a “temporary moment of time.”

The president made a point of referring to it as a “foreign virus” and blamed the European Union for having “failed to take the same precautions” as he had in limiting travel from China, where the outbreak got its start, even though Europe has not been a major source of known infections so far in the United States.

Mr. Trump imposed a ban on foreigners who have been in the 26 countries that make up Europe’s Schengen Area in the previous 14 days. The limits take effect Friday at midnight for 30 days but exempt American citizens, permanent legal residents and their families, although they could be funneled to certain airports for enhanced screening. The State Department also advised Americans to “reconsider travel abroad” anywhere because of the coronavirus.

In the course of his speech, Mr. Trump mischaracterized his own policies, even though he was reading from the teleprompter. He said he was “suspending all travel from Europe to the United States,” when in fact it applies only to foreigners, and he said it would “apply to the tremendous amount of trade and cargo” across the ocean, when in fact his order affects only people. He corrected the latter afterward on Twitter, saying “The restriction stops people not goods.”

Mr. Trump also promised aid to workers who are ill, quarantined or caring for others because of the coronavirus and asked Congress for $50 billion to extend that aid. He said he would expand low-interest Small Business Administration loans in affected areas and authorize the deferral of up to $200 billion in tax payments for certain affected people and businesses. But he remained at loggerheads with Congress on more comprehensive measures.

Governors chose not to wait for Washington. Gov. Andrew M. Cuomo of New York said that the state-run university system and the City university and colleges, with about 700,000 students, would shift primarily to online classes starting on March 19.

Gov. Jay Inslee of Washington State, where 29 coronavirus patients have died, banned any gatherings of 250 or more people in three counties, while Seattle’s schools will close on Thursday for two weeks. Gov. Andy Beshear of Kentucky urged churches to cancel services this weekend, a major cultural moment in a state with a deeply religious population.

Other countries expanded their own actions. India suspended virtually all visas to enter the country until April 15. Iraq and Lebanon canceled Friday prayers. Greece and Ukraine are closing all schools, universities and kindergartens. Hungary is closing universities and theaters. Poland is closing theaters, museums and art galleries. And Denmark is suspending naturalization ceremonies because a handshake is officially required for their completion.

With the worst outbreak in Europe causing more than 800 deaths, Italy has already all but ordered its entire population to stay at home. The government reported a jump of 2,000 more infections on Wednesday, to a total of more than 12,460. “Today, the red zone is Italy,” former Prime Minister Matteo Renzi said, but if containment fails, “the red zone will be Europe.”

Ms. Merkel made her first public appearance addressing the outbreak in a country with 1,600 infected. “Given a virus for which there is no immunity and no immunization, we have to understand that many people will be infected,” she said. “The consensus among experts is that 60 to 70 percent of the population will be infected.”

She added: “How we respond matters. We are playing for time.”

Ms. Merkel’s counterpart in France — which has about 2,300 confirmed cases — President Emmanuel Macron, was equally grim after a meeting where European leaders decided to set up a $28 billion investment fund but failed to overcome disagreements about sharing medical equipment like face masks and respirators. “What we are living is a true world crisis,” he said.

The toll of the health crisis on the economic prospects of the United States and other countries was clear by the time markets closed on Wednesday afternoon. The S&P 500 closed 19 percent down from its peak last month, and the Dow Jones industrial average 20 percent down from its peak, officially ending the bull market that began after the financial crash of 2008.

After weeks of sports events getting canceled in piecemeal fashion, a dam broke Wednesday as organizers announced previously unimaginable cancellations and changes to some of the biggest events on the calendar.

Within hours of the testimony from Dr. Fauci, there was a string of cancellations, culminating with the announcement that the N.C.A.A. would hold its men’s and women’s basketball tournaments without fans and only essential personnel and their families present followed by the N.B.A. suspension.

Likewise canceled were ski races in Sweden, figure skating’s world championships in Montreal, and all international sporting events in Argentina until the end of March. But organizers of the Summer Olympics in Tokyo insisted that they still planned to hold the games in late July despite a suggestion by a member of the local committee that it delay for a year or two.

Late-night comics like Stephen Colbert, Jimmy Fallon, Trevor Noah and Samantha Bee decided to tape their shows without live audiences. More businesses ordered employees to work from home, including The New York Times. And Washington National Cathedral suspended services for two weeks, offering instead a “virtual service” online.

After adamantly refusing to cancel campaign rallies or alter his own schedule, Mr. Trump on Wednesday night scrubbed plans to travel to Las Vegas for a conference of the Republican Jewish Coalition, which then promptly canceled its event.

At the White House, the president huddled with banking executives to discuss a way forward after congressional leaders reacted coolly to his suggestions for a payroll tax cut. With the White House and Democrats divided over what a broader economic stimulus package, the two parties appeared to coalesce around a narrower short-term bill focusing on paid leave, enhanced unemployment insurance, food assistance and help for small businesses, deferring the rest until Congress returns from a weeklong recess.

Previewing part of the president’s plan, Treasury Secretary Steven Mnuchin said the Internal Revenue Service would delay tax payments without penalty or interest for “virtually all Americans other than the superrich.”

Testifying on Capitol Hill, Mr. Mnuchin noted that all individuals were allowed to request tax filing extensions online, but that this would be a special provision to help small and midsize businesses and “hardworking individuals” afflicted by the coronavirus. The delay would not apply to large corporations or the wealthiest Americans, he said, without elaborating.

It fell to Dr. Fauci and other health officials to sound the alarm that Mr. Trump seemed unwilling to voice himself. “We must be much more serious as a country about what we might expect,” Dr. Fauci told the House Oversight and Reform Committee. “We cannot look at it and say, ‘Well, there are only a couple of cases here, that’s good.’ Because a couple of cases today are going to be many, many cases tomorrow.”

Dr. Robert R. Redfield, the director of the Centers for Disease Control and Prevention, said that “this is the time for everyone to get engaged.”

The hearing quickly devolved into a partisan fight over the Trump administration’s handling of the outbreak. Democrats assailed the president for a slow response and misleading statements, while Republicans said Mr. Trump’s critics were trying to score political points.

“The 24/7 criticism the president is undergoing is unwarranted at a minimum,” said Representative Mark Green, Republican of Tennessee.

Representative Gerald E. Connolly, Democrat of Virginia, displayed a picture of the president wearing a campaign hat while visiting the C.D.C. “We will not be lectured about politicization,” he said, “and all of your words and sanctimony will not cover up the fact that this administration was not prepared for this crisis and put lives, American lives, at risk.”

Reporting was contributed by Elisabetta Povoledo, Steven Erlanger, Alissa J. Rubin, Annie Karni, Marc Santora, Megan Specia, Vindu Goel, Elian Peltier, Jason Horowitz, Emma Bubola, Nicholas Bogel-Burroughs, Jorge Arangure, Matthew Futterman, Elaine Yu, Amy Qin, Alan Rappeport, Emily Cochrane, Noah Weiland, Sheri Fink, Mike Baker, Monika Pronczuk, Melissa Eddy, Roni Caryn Rabin, Donald G. McNeil Jr., Andrew Keh, and Katie Thomas.