The S&P 500 and Dow both fell more than 3 percent on Thursday, with investors worried about the outlook for the U.S. economy and the debt crisis in Europe.

The Dow Jones industrial average .DJI was down 351.48 points, or 2.95 percent, at 11,544.96, after briefly falling 3 percent. The Standard & Poor's 500 Index .SPX was down 42.05 points, or 3.34 percent, at 1,218.29. The Nasdaq Composite Index .IXIC was down 94.45 points, or 3.51 percent, at 2,598.62.