[Updated at 4:16 p.m.] The Dow Jones Industrial Average has suffered its worst one-day loss since December 2008, plummeting 634.76 points - to 10,809.85 - after S&P downgraded its rating for U.S. credit.
Monday's preliminary figure, after trading ending at 4 p.m. ET, represents a one-day loss of 5.5%. The Dow is below 11,000 for the first time since November.
The S&P 500 fell 6.6% and Nasdaq Composite was down around 6.9%.
[Updated at 2:27 p.m.] Dow extends loss to 600 points as stock sell-off accelerates.
[Updated at 2:03 p.m.] Dow drops 450 points, falling below 11,000 for first time since last November, as sell-off extends.
[Updated at 1:57 p.m.] President Barack Obama said Monday that the downgrade of the U.S. credit rating by Standard & Poor's occurred because "they doubted our political system's ability to act."
[Updated at 10:33 a.m.] Stocks extend losses after Mortgage finance agencies Freddie Mac and Fannie Mae were downgraded by S&P, following downgrade of the United States. Dow falls more than 300 points.
[Updated at 10:09 a.m.] U.S. stocks plunged Monday as Wall Street had its first opportunity to react to Standard & Poor's downgrade of U.S. debt.
The Dow Jones industrial average was down 203 points, or 1.8%; the S&P 500 lost 25 points, or 2%; and the Nasdaq Composite dropped 58 points, or 2.8%.
Monday's selloff was broad, with 28 out of the Dow 30 and 495 members of the S&P 500 trading in the red. The blue chips were dragged lower by a 8% drop in Bank of America shares and a 4% decline in shares of Alcoa.
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[Posted at 9:12 a.m.] U.S. stocks were set to tumble at the open Monday, tracking deep declines in global markets, as investors get their first opportunity to react to Standard & Poor's downgrade of U.S. debt.
S&P 500 and Nasdaq futures were all down more than 2% ahead of the opening bell. Dow Jones industrial average futures were less than 2% lower. Stock futures indicate the possible direction of the markets when they open at 9:30 a.m. ET.
Late Friday, S&P downgraded United States' credit rating by one notch to AA , removing it from the Triple A-club for the first time in history.
Economists and investment strategists largely agree that there could be an initial shock, but the downgrade itself should not impact markets too much.
Still, the rating cut is unprecedented, so nobody can be certain.FULL CNNMONEY.COM STORY