The selling on Wall Street began at the opening bell on Monday and only intensified as the day went on. Shares moved sharply lower as the banking crisis tightened its grip on the global economy.

The Dow Jones industrial average fell below 10,000 for the first time since 2004 after losing more than 500 points in the first hour. The index has lost more than 1,100 points — or about 10 percent — in slightly more than a week.

Shortly after noon, the Dow was down 524 points or 5 percent.

The broader American stock market was down more than 5.7 percent, as measured by the Standard & Poor’s 500-stock index, its worst decline since last Monday’s 8.8 percent drop. At the same time, oil dropped below $90 a barrel.

The precipitous declines on Wall Street came a day after European governments were forced to scramble to save several major banks and lenders from collapse. The moves reinforced the global reach of the current crisis and alarmed depositors and regulators in the United States and abroad.