Stocks extended losses in heavy volume on Monday, with the S&P 500 down more than 6 percent, in the first session since Standard & Poor's cut the United States' top-tier AAA credit rating.

The Dow Jones industrial average dropped 554.08 points, or 4.84 percent, to 10,890.53. The Standard & Poor's 500 Index fell 73.44 points, or 6.12 percent, to 1,125.94. The Nasdaq Composite Index lost 155.96 points, or 6.16 percent, to 2,376.45.

With more than an hour of trading left, more than 11.5 billion shares traded on the New York Stock Exchange, NYSE Amex and Nasdaq, above last year's estimated daily average of 8.47 billion.

Declining stocks outnumbered advancing ones on the NYSE by a ratio of about 64 to 1.

(Editing by Kenneth Barry)