The S&P 500 closed down about 9.5 percent, its biggest daily drop since the stock market crashed in 1987, on what came to be known as Black Monday. The decline has left stocks in the United States firmly in a bear market — a term that signifies a decline of 20 percent from the most recent highs.

For the Dow Jones industrial average, the drop of 10 percent was also its worst since the 1987 stock market crash.

The travel ban hit shares in Europe particularly hard, with major stock indexes there down more than 10 percent. It also battered airline stocks. And, with oil prices falling, energy companies were among the day’s biggest losers, too.

Waves of selling have come as investors have been dismayed by Washington’s inability to rally around a meaningful response to the economic toll the outbreak will take. Mr. Trump said on Wednesday that he would extend financial relief for sick workers and would ask Congress for more. But he remained at loggerheads with Congress on more comprehensive measures.

The most notable step Mr. Trump announced on Wednesday, that the United States would stop travel from most European countries outside Britain for 30 days, hurt financial markets more than it helped. And on Thursday, he said he could restrict domestic travel to regions of the United States when the coronavirus becomes “too hot.”