NEW YORK — A stunning makeover of the Wall Street landscape sent stocks falling precipitously Monday, with the Dow Jones industrials sliding 500 points in their worst point drop since the September 2001 terrorist attacks. Investors reacted badly to a shakeup of the financial industry that took out two storied names: Lehman Brothers Holdings Inc. and Merrill Lynch & Co.

Stocks also posted big losses in markets across much of the globe as investors absorbed Lehman's bankruptcy filing and what was essentially a forced sale of Merrill Lynch to Bank of America for $50 billion in stock. While those companies' situations had reached some resolution, the market remained anxious about American International Group Inc., which is seeking emergency funding to shore up its balance sheet. A faltering of the world's largest insurance company likely would have financial implications far beyond that of Lehman, the largest U.S. bankruptcy.

The swift developments that took place Sunday are the biggest yet in the 14-month-old credit crises that stems from now toxic subprime mortgage debt. For the first part of Monday's trading, the market was falling, but in a largely orderly fashion as investors seemed to draw some relief from the resolution of Lehman's problems.

But as the session wore on, and there was no word about AIG, the market's suffered another bout of fear that the ongoing credit crisis will continue to devastate the financial sector, and selling accelerated in the final hour.

Investors are worried that trouble at AIG and the bankruptcy filing by Lehman, felled by $60 billion in bad debt and a dearth of investor confidence, will touch off another series of troubles for banks and financial institutions that may be forced to further write down the value of their own debt assets. Wall Street had been hopeful six months ago that the collapse of Bear Stearns would mark the darkest day of the credit crisis.

AIG's troubles a week after its stock dropped 45 percent are worrisome for some investors because of the company's enormous balance sheet and the risks that troubles with that companies finances could spill over to the companies with which it does business. AIG, one of the 30 stocks that make up the Dow industrials, fell $6.93, or 57 percent, to $5.21 Monday as investors worried that it would be the subject of downgrades from credit ratings agencies.

According to preliminary calculations, the Dow fell 504.48, or 4.42 percent, to 10,917.51, moving below the 11,000 mark for the first time since mid-July. It was the worst point drop for the Dow since it lost 684.81 on Sept. 17, 2001, the first day of trading after the terror attacks. It was also the sixth-largest point drop in the Dow, just behind the 508.00 it suffered in the October 1987 crash.

Broader stock indicators also fell. The Standard & Poor's 500 index declined 58.74, or 4.69 percent, to 1,192.96, and the Nasdaq composite index fell 81.36, or 3.60 percent, to 2,179.91.

The S&P 500 broke through the 1,200.44 trading low seen in mid-July, a key level traders watch. Much of the trading day until about the last hour had been orderly because the market had tested another key level early in the session and managed to stay above it. But the eventual drift lower prompted some investors to hit the "sell" button.