The S&P 500 is headed for its biggest annual decline since the Great Depression, when it fell 47 percent in 1931.

``The final low will be much lower than this,'' and may not occur before the fourth quarter of next year, de Graaf said.

At a minimum, stocks are likely to revisit their lowest levels of 2002 and 2003, when a 51 percent slide from the March 2000 peak sent the S&P 500 as low as 768.63, said Mary Ann Bartels, chief market analyst at Merrill Lynch & Co. in New York.

The S&P 500 is likely to fall to around 680, 20 percent lower than yesterday's close, probably by the end of the year, said John Roque, senior technical analyst for New York-based brokerage Natixis Bleichroeder Inc.