Fearing that the crisis in the financial industry could stun the broader economy, investors drove stocks down almost 5 percent Monday, sending the Dow Jones industrial average and Standard & Poor’s 500-stock index to their lowest levels in two years.

The Dow fell 504.48 points, its biggest one-day point drop since Sept. 17, 2001, the first trading day after the Sept. 11 terrorist attacks.

In the minutes before the opening of the New York Stock Exchange, dozens of traders were clustered around the specialists who oversee trading of American International Group and Bank of America, shouting bids and offers. As the opening bell clanged, dozens of flat-panel monitors around the specialists’ posts pulsed with frantic trading.

With Lehman filing for bankruptcy and A.I.G. in distress, investors were worried that consumers and companies would have difficulty getting loans.