Stocks plunged in the final hour of trading Thursday, sending the Dow Jones industrial average down 679 points — more than 7 percent — to its lowest level in five years after a major credit ratings agency said it might cut its rating on General Motors Corp.

The Standard & Poor’s 500 index also fell more than 7 percent.

The declines came on the one-year anniversary of the closing highs of the Dow and the S&P. The Dow has lost 5,585 points, or 39.4 percent, since closing at 14,198 on Oct. 9, 2007. The S&P 500, meanwhile, is off 655 points, or 41.9 percent, since recording its high of 1,565.15.

U.S. stock market paper losses totaled $872 billion Thursday and the value of shares overall has tumbled a stunning $8.33 trillion since last year’s high. That’s based on preliminary figures measured by the Dow Jones Wilshire 5000 Composite Index, which tracks 5,000 U.S.-based companies’ stocks and represents almost all stocks traded in America.

Thursday’s sell-off came as Standard & Poor’s Ratings Services put GM and its finance affiliate GMAC LLC under review to see if its rating should be cut. GM has been struggling with weak car sales in North America.

The action means there is a 50 percent chance that S&P will lower GM’s and GMAC’s ratings in the next three months.

S&P also put Ford Motor Co. on credit watch negative. The ratings agency said that GM and Ford have adequate liquidity now, but that could change in 2009.

GM led the Dow lower, falling $2.15, or 31 percent, to $4.76, while Ford fell 58 cents, or 22 percent, to $2.08.

“The story is getting to be like that movie ’Groundhog Day,”’ said Arthur Hogan, chief market analyst at Jefferies & Co. He pointed to the still-frozen credit markets, and Libor, the bank-to-bank lending rate that remains stubbornly high despite interest rate cuts this week by the Federal Reserve and other major central banks.

“Until that starts coming down, you’ll be hard-pressed to find anyone getting excited about stocks,” Hogan said. “Everything we’re seeing is historic. The problem is historic, the solutions are historic, and unfortunately, the sell-off is historic. It’s not the kind of history you want to be making.”

The Dow ended the day at its lows, finishing down 678.91, or 7.3 percent, at 8,579.19. The blue chips hadn’t closed below the 9,000 level since the June 30, 2003.

Broader stock indicators also tumbled. The S&P 500 fell 75.02, or 7.6 percent, to 909.92, while the Nasdaq composite index fell 95.21, or 5.5 percent, to 1,645.12.

The Russell 2000 index of smaller companies fell 47.37, or 8.7 percent, to 499.20.

A wave of fear about the economy sent stocks lower in the final two hours of trading after a volatile morning in which major indicators like the Dow and the S&P 500 index bobbed up and down. The Nasdaq, with a bevy of tech stocks, spent much of the session higher but eventually declined as the sell-off intensified. Still, its losses were less severe because of the relatively modest drops in names like Intel Corp. and Microsoft Corp.

On the New York Stock Exchange, declining issues came to nearly 3,000, while fewer than 250 advanced.