Oct. 24 (Bloomberg) -- Trading in futures on the Standard & Poor's 500 Index, the Dow Jones Industrial Average and the Nasdaq 100 Index was limited until U.S. markets open to stop the contracts from extending losses of more than 6 percent today. The S&P 500 futures will not trade below 855.20, the so- called limit down level this quarter, until U.S. regular trading begins at 9:30 a.m. New York time, said Jeremy Hughes, a London- based spokesman for the Chicago Mercantile Exchange. Dow Average futures won't trade below the 8,224 level, he said, while the Nasdaq 100 futures won't fall below 1,168.50. The ``limit down'' suspension allows the contracts to trade above those levels, he said.