US stocks plummeted at the close of business on the first trading day since Standard & Poor's downgraded American debt.
The Dow Jones closed down 634 points, or 5.5%, at 10,809 - the first time it has fallen below 11,000 since November and its biggest one-day point drop since December 2008.
The S&P 500 is closing down 79, or 6.7%, at 1,119.46. The Nasdaq is down 174.72, or 6.9%, at 2,357.69.
Investors are worried about the slowing US economy and escalating debt problems threatening Europe.
Money flowed out of stocks and into safer investments, including gold.