The Dow Jones Industrial Average plunged 634.76 points, or 5.55 percent, to finish at 10,809.85, well below the psychologically-significant 11,000 mark. The move marks the blue-chip index's biggest point and percent drop since Dec. 1, 2008.
BofA [BAC 6.51 -1.66 (-20.32%) ] and Alcoa [AA 11.33 -1.46 (-11.42%) ] were the top laggards on the index.
The S&P 500 plummeted 79.92 points, or 6.66 percent, to close at 1,119.46, its lowest close since Sept. 10, 2010.
Nasdaq sank 174.72 points, or 6.90 percent, to end at 2,357.69, its lowest close since October 4, 2010.
August is already on track to be the worst month for the S&P and Nasdaq since Oct. 2008.
No one knows what tomorrow will bring.The CBOE Volatility Index, widely considered the best gauge of fear in the market, spiked above 40 to touch its highest level since Mar. 2009.