The S&P 500 and Nasdaq fell more than 4 percent on much higher-than-average volume while the Dow added to losses on Thursday, as worries about the outlook for the U.S. economy and the debt crisis in Europe escalated.

The S&P 500 was on track for its biggest daily percentage drop since April 20, 2009.

The Dow Jones industrial average was down 436.21 points, or 3.67 percent, at 11,460.23. The Standard & Poor's 500 Index was down 51.32 points, or 4.07 percent, at 1,209.02. The Nasdaq Composite Index was down 118.51 points, or 4.40 percent, at 2,574.56.

(Reporting by Caroline Valetkevitch, Editing by Chizu Nomiyama)