The Dow Jones industrial average fell 161.83 to 7,872.82 in the first 30 minutes of trading.
Broad-market measures also slid about 2 percent, and declining issues outnumbered advancers by a 9-to-1 margin on the New York Stock Exchange.
And overseas, the Nikkei stock average fell 3.0 percent in Tokyo, Frankfurt's DAX index was down 4.6 percent and London's FT-SE 100 was down 3.4 percent.
U.S. Treasury bonds, however, shot higher as investors at home and abroad sought out the perceived safety of fixed-income securities and a stable U.S. economy.
The worldwide downturn was triggered by a shocking 10 percent slide in the Hang Seng index of Hong Kong that aggravated worries about companies that depend on Asian markets for a big portion of their profits.
Today's downturn was the latest setback in the U.S. stock market's struggle to fully recover from a steep slide in August.
The Standard & Poor's 500 list was down 18.15 today at 950.34, and the NYSE composite index was down 9.25 at 499.55.
The Nasdaq composite index was down 39.67 at 1,668.41, and the American Stock Exchange composite index was down 9.65 at 704.09.
On Wednesday, Boeing and AlliedSignal spoiled this week's stream of
reassuring profit reports, but stocks suffered only minor losses as the
overall trend in third-quarter results remained positive.