NEW YORK—Wall Street hit levels not seen since 2003 on Wednesday, with the Dow Jones industrial average plunging below the 8,000 mark as the fate of Detroit’s Big Three automakers amid a slumping economy disheartened investors.

A cascade of selling occurred in the final minutes of the session as investors yanked money out of the market. For many, the real fear is that the recession might be even more protracted if Capitol Hill is unable to bail out the troubled auto industry.

Investors also scoured economic data that included minutes from the last meeting of the Federal Reserve in which policymakers lowered projections for economic activity this year and next. Economic worries caused across-the-board selling, with financial stocks particularly hard hit.

The S&P 500, widely considered the broadest snapshot of corporate America, slipped 6.12 percent to 806.58; and the Dow gave up 5.07 percent to 7,997.28. Both closed at their lowest levels since March 2003.

The financial crisis has already wiped out $6.69 trillion of value from the S&P 500 since its October 2007 high, and many fear more is to come. Stocks have traded with high volatility in the past few months, with the major indexes soaring only to plunge an hour later as the market looks for a bottom.

“I don’t know what the catalyst is going to be where we turn the corner and people start buying stocks wholeheartedly again,” said Jon Biele, head of capital markets at Cowen & Co. “People got out of the way. The financial situation hasn’t changed.”

The selling on the New York Stock Exchange was staggering—only 158 companies that trade

there finished the day positive while 2,943 declined. Volume again was light, a symptom of the market’s recent volatility, with 1.63 million shares exchanging hands by the close.

Smaller stocks also got clobbered. The Russell 2000 index gave up 35.13, or 7.85 percent, to 412.38.