(Updates with trading curbs)

NEW YORK, July 26 (Reuters) - The New York Stock Exchange said on Tuesday it instituted downside trading curbs at 11:06 a.m. (1506 GMT) as U.S. stocks fell sharply on concerns about deterioration in the credit markets.

The New York Stock Exchange Composite Index .NYA, which is used to determine when to begin trading curbs, was down 205.45 points, or 2.07 percent, at 9,724.47.

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