U.S. stocks plunged at the open Monday as Wall Street had its first opportunity to react to Standard & Poor's downgrade of U.S. debt.

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U.S. stocks plunged at the open Monday as Wall Street had its first opportunity to react to Standard & Poor's downgrade of U.S. debt.

At the open, the Dow Jones industrial average was down 184 points, or 1.6%; the S&P 500 lost 30 points, or 2.5%; and the Nasdaq Composite dropped 82 points, or 3.2%.

Late Friday, S&P downgraded the United States' credit rating by one notch to "AA ", removing the world's largest economy from the Triple A-club for the first time in history.

Economists and investment strategists said an initial shock is not surprising - global markets plunged on the news as well  - but emphasized that the downgrade itself should not impact markets too much.