U.S. stocks fell to session lows on Tuesday after the credit ratings for both Greece and Portugal were cut, intensifying worries over their sovereign debt.

* The CBOE Volatility Index <.VIX>, known as Wall Street's fear gauge, jumped 17 percent following the downgrade of Greek ratings into junk territory.

* The Dow Jones industrial average <.DJI> dropped 146.61 points, or 1.31 percent, to 11,058.42. The Standard & Poor's 500 Index <.SPX> lost 19.30 points, or 1.59 percent, to 1,192.75. The Nasdaq Composite Index <.IXIC> gave up 34.53 points, or 1.37 percent, at 2,488.42.

(Reporting by Leah Schnurr; editing by Jeffrey Benkoe)