The Dow Jones industrial average fell more than 200 points Thursday, its biggest decline in six weeks.
The pullback came as investors reacted to discouraging economic reports from China and intensifying tensions in Ukraine.
It was the worst day for the market in six weeks and the fourth loss in a row for the Dow. The plunge was a sharp contrast to the relatively quiet trading earlier in the week following a record-setting run last week.
Stocks started the day trading slightly higher following news of a pickup in U.S. retail sales last month, but the gains didn't last.
"The data out of China has been weak. The retail sector in America seems to be a total disaster. It's enough, combined with what's going on in Ukraine, to get people a little bit nervous and sell," said Ian Winer, director of trading at Wedbush Securities.
The Dow Jones industrial average slid 231.19 points, or 1.4 percent, to 16,108.89. The S&P 500 index fell 21.86 points, or 1.2 percent, to close at 1,846.34. The Nasdaq composite dropped 62.91 points, or 1.5 percent, to 4,260.42.
The last time the market had a bigger decline was Feb. 3, when the Dow sank 326 points, or 2.1 percent.
Thursday's slide erased the S&P 500 index's gains for the year and extended the Dow's year-to-date loss to 2.8 percent. The Nasdaq is still up 2 percent so far this year.
Stocks that fell outnumbered those that rose more than two to one.
Bond prices rose as traders sought safety. The yield on the 10-year Treasury note declined to 2.65 percent from 2.73 percent a day earlier as bond prices rose.
Nine of the 10 sectors in the S&P 500 index fell. Information technology lost the most. Utilities bucked the trend, rising 0.9 percent. Investors tend to buy those stocks when they want to reduce risk and hold stable companies that pay steady dividends.
Concerns over China worsened Thursday after government figures there showed industrial production rose in the first two months of the year at a rate that was lower than analysts were expecting. Retail sales growth also fell short of estimates.
"At this stage, investors are linking these negative data points coming out of China and they don't like what they see," said Lawrence Creatura, a portfolio manager at Federated Investors. "Even small hiccups there can have large implications for investors."
The market jitters intensified later in the morning, when President Barack Obama issued remarks after meeting with Ukraine's new prime minister at the White House.
Obama said that if Russia continues an aggressive path in Ukraine, the United States and other countries will be "forced to apply costs" to Moscow.
Citizens in the Ukrainian region of Crimea are set to vote on joining Russia on Sunday. The U.S. and European Union say the referendum violates Ukraine's constitution and international law. Russia has said it will respect the results.
Secretary of State John Kerry told a Senate committee on Thursday that Moscow should expect the U.S. and Europe to take measures against it should Russia act on a vote by Crimea to join Russia.
"The hardening of the rhetoric in these communications is a change," Creatura said.
Winer said that investors weren't panicked.
"The selling is pretty complacent," he said. "This is more about how people are positioned in the market."
Several companies that provide oil and gas offshore drilling services fell Thursday.
Diamond Offshore Drilling fell $1.99, or 4.3 percent, to $44.39, while Noble Corp. shed $1.38, or 4.5 percent, to $28.98. Transocean lost $1.25, or 3.1 percent, to $39.54, and National Oilwell Varco slid $2.13, or 2.8 percent, to $75.18.
Investors received some encouraging news on the U.S. employment picture.
The government reported that applications for unemployment benefits dropped 9,000 last week to 315,000. Applications are a rough proxy for layoffs. The declines indicate companies are confident enough about the economy to keep their staffs.
A separate government report showed U.S. retail sales rose 0.3 percent in February as Americans spent more on autos, clothing and furniture.
Spending had fallen 0.6 percent in January. The increase suggests that spending has started to recover after being tempered by snowstorms and freezing temperatures that blanketed much of the country.
Among the stocks that posted gains was Plug Power Inc. The alternative energy company reported revenue jumped to $8 million as it lined up some big clients. The news sent the stock vaulting up $1.20, or 17.6 percent, to $8. The stock has been soaring in recent weeks, and traded below 50 cents a share as recently as Nov. 6.