The Dow Jones Industrial Average plunged 4.3 percent Thursday, its worst one-day drop in more than two years, as global markets melted down over fears of another world economic downturn.
The Dow was down 512.76 points to 11,383.68; the broader S&P 500 lost 4.8 percent to 1,200.07, while the tech-heavy Nasdaq Composite plunged 5.1 percent to 2,556.39.
More turmoil over sovereign debt problems in Europe and feeble US economic data are stoking "fear that the economy is heading for a double-dip recession," said Peter Cardillo of Rockwell Global Capital.
"The market is pricing that in," he said.
Markets worldwide were on edge over fiscal weakness in Italy and Spain and the eurozone's ability to contain more crisis, as the two countries' borrowing costs surged in recent days.
Meanwhile the US Labor Department reported that weekly claims for unemployment benefits remained at a high 400,000 last week, as business and government layoffs persisted while new job creation remained sluggish.
All of the Dow's 30 blue-chip stocks were hit by the sell-off, but losses were most pronounced in the basic materials sectors, energy and financial companies.