NEW YORK--The Dow Jones industrial average tumbled 550 points yesterday, forcing the stock market to shut down for the first time since the 1981 assassination attempt on President Reagan.

The market's best known barometer fell 554.26 points to 7,161.15, surpassing the 508-point Black Monday crash of 1987 as its biggest point drop ever.

But on a percentage basis, yesterday's 7.18 percent drop by the Dow only ranked as the 12th biggest ever and didn't come close to the 22 percent loss on Oct. 19, 1987.

Although the Dow is still up 11 percent since the beginning of the year, the sell-off put the Dow's losses at about 900 points over the past four sessions and 1,100 points since it set a record high at 8,259.31 on Aug. 6.

"It's a bloodbath," said Arnold Kaufman, a market analyst at Standard & Poor's. "It scares you because when you get a decline this fast, there's a risk it will keep snowballing."

The Dow's drop triggered two circuit breakers on the New York Stock Exchange that had never been set off since they were put in place after the 1987 sell-off. The first circuit breaker, at 350 points, closed the market for 30 minutes. The second, at 550, halted trading for the day.

It remains to be seen how much of yesterday's selling was fueled by mutual fund investors. Publicly, however, many individual investors portrayed an unflappable facade.

"I wish I had more money to invest," said Helen Ginty, 60, a secretary in New York, asserting that she plans no change in her retirement investment strategy. "I don't think people are as crazy as they used to be" during a downturn.

For many analysts, the drop was notable because the Dow has now fallen 13.3 percent from its Aug. 6 record high of 8,259.31, its first downturn of at least 10 percent in seven years, the longest such streak since the 1960s.

Declining issues also outnumbered advancers by an astounding 16-to-1 margin on the New York Stock Exchange, where volume came to 685.50 million shares, the busiest day ever in the exchange's history.

Stocks started the day lower as another sharp sell-off in Hong Kong triggered another wave of selling in financial markets around the globe, but the selling didn't pick up steam in the United States until yesterday afternoon. The Dow, for example, was down just 115 points at midday. midday.

The mounting financial crisis in Hong Kong has ignited fears about whether global business conditions will be undermined by Southeast Asia's shaky economics, where mounting trade deficits have sent interest rates soaring and local currencies plunging.

Hong Kong's Hang Seng index fell 7 percent, as foreign markets also fell yesterday.

S&P's Kaufman, however, asserted that a stream of weak earnings reports or a jump in interest rates could have set off a steep decline just as easily as Hong Kong's problems.

"The Southeast Asia crisis is just a trigger," he said. "Something would have come along at some point. This is what came along."

And once again, analysts stressed that although plenty of U.S. companies do business in Asia, this nation's financial health is only slightly dependent on the fortunes of Southeast Asia.

"I take great heart in the fact that fundamentally, the underpinning of our market is solid," said John Shaughnessy, chief investment strategist at Advest Inc. in Hartford, Conn