NEW YORK -- U.S. stocks fell sharply at the open Thursday on more signs of turmoil in the subprime mortgage sector, sending the New York Stock Exchange Composite Index down more than 190 points, past the level that triggers the NYSE to impose trading curbs.

The curbs require that all program selling of S&P 500 stocks must be on an up-tick.

NYSE was not available to confirm whether trading curbs had been imposed.

The NYSE composite was down 198.55 points.

The Dow Jones industrial average was down 184.04 points, or 1.35 percent, at 13,473.82. The Standard & Poor's 500 Index was down 24.33 points, or 1.62 percent, at 1,473.16. The Nasdaq Composite Index was down 28.27 points, or 1.08 percent, at 2,584.71.

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