Another dose of disappointing economic news sent the Dow industrials down nearly 270 points.
The Dow closed down 269.50, or 3.2%, at 8,043.63, while the Nasdaq dropped 41.91, or 3.4%, to 1,206.01 - a new five-year low.
The Standard & Poor's lost 29.64, or 3.4%, to 834.60, and the Russell 2000 fell 9.33 to 367.12.
It last closed lower on April 21, 1997, when it stood at 1,203.95.
Monday's selling followed a report from the Institute for Supply Management that showed the US service sector grew in July, but at a slower pace than many economists had predicted. The group's non-manufacturing index stood at 53.1 in July, after standing at 57.2 in June and 60.1 in May.
Procter & Gamble fell $2.40 to $87.44 despite fourth-quarter results that exceeded Wall Street's expectations, but failed to impress investors who had hoped for more.
Financial stocks were also lower. That included Citigroup, which slid $2.23, or 7.2%, to $28.65, following a cautious note about the company's prospects from Lehman Brothers.
Among tech stocks, Texas Instruments dropped $1.77 to $18.20. Cisco Systems, which is expected to report earnings after the market closes tomorrow, tumbled 53 cents, or 4.5%, to $11.36.
Retailers also retreated. Wal-Mart fell 50 cents to $45.60, while JC Penney lost 10 cents to $15.91.
Declining issues led advancers 3 to 1 on the New York Stock Exchange. Volume was brisk.