Investors fled the U.S. stock market on Tuesday and the S&P 500 tumbled to nearly an eight-month low in a sell-off triggered by a wave of increasing alarm over the global economic outlook.

The S&P 500 broke a key technical resistance level as it hit a session low below 1,040.78, its previous intraday low for 2010.

The Dow Jones industrial average <.DJI> fell 268.22 points, or 2.65 percent, to finish unofficially at 9,870.30. The Standard & Poor's 500 Index <.SPX> dropped 33.33 points, or 3.10 percent, to close unofficially 1,041.24. The Nasdaq Composite Index <.IXIC> slid 85.47 points, or 3.85 percent, to end unofficially at 2,135.18.

(Reporting by Rodrigo Campos; Editing by Jan Paschal)