NEW YORK (AP) — Stocks had their worst day of trading since the September 11, 2001 terrorist attacks, briefly hurtling the Dow Jones industrials down more than 500 points on a world-wide tide of concern that the US and Chinese economies are stumbling and that share prices have become over-inflated.The steepness of the market’s drop, as well as its global breadth, signalled a possible correction after a long period of stable and steadily rising stock markets, which had not been shaken by such a volatile day of trading in several years.

A nine percent slide in Chinese stocks, which came a day after investors sent Shanghai’s benchmark index to a record high close, set the tone for US trading.

The Dow began the day falling sharply, and the decline accelerated throughout the course of the session before stocks took a huge plunge in late afternoon as computer-driven sell programs kicked in.

The Dow fell 546.02, or 4.3 percent, to 12,086.06 before recovering some ground in the last hour of trading to close down 416.02, or 3.29 percent, at 12,216.24, according to preliminary calculations.

Because the worst of the plunge took place after 2:30 p.m., the New York Stock Exchange’s trading limits, designed to halt such precipitous moves, were not activated.

The decline was the Dow’s worst since September 17, 2001, the first trading day after the terror attacks, when the blue chips closed down 684.81, or 7.13 percent.

The drop hit every sector of stocks across the market. Riskier issues such as small-cap and technology stocks suffered the biggest declines.

But analysts who have been expecting a pullback after a huge rally that began last October and sent the Dow to a series of record highs, were unfazed by Tuesday’s drop.

“This corrective consolidation phase isn’t just going to be one day, but we don’t believe this is going to be a bear market,” said Bob Doll, BlackRock’s global chief investment officer of equities.

Some investors also tried to put Tuesday’s slide into a longer-term perspective.

“All who invest should feel grateful that we’ve had a great run for the last 12 to 18 months,” said Joel Kleinman, a Washington, D.C. attorney, adding that he has learned to not read too much into any short-term ups and downs. “This is another day in the market.”

Still, traders’ dwindling confidence was knocked down further by data showing that the economy may be decelerating more than anticipated. A Commerce Department report that orders for durable goods in January dropped by the largest amount in three months exacerbated jitters about the direction of the US economy, just a day after former Federal Reserve Chairman Alan Greenspan said the United States may be headed for a recession.

“It looks more and more like the economy is a slow growth economy,” said Michael Strauss, chief economist at Commonfund. “Moderate economic growth is good — an abrupt stop in economic growth scares people.”

The market had been expecting the government on Wednesday to revise its estimate of fourth-quarter GDP growth down to an annual rate of about 2.3 percent from an initial forecast of 3.5 percent, and grew increasingly nervous that the figure could come in even lower.

The housing market, which the Street had been hoping had bottomed out, also looked far from recovery after a Standard & Poor’s index indicated that single-family home prices across the nation were flat in December.

A later report from the National Association of Realtors said existing home sales climbed in January by the largest amount in two years, but the data didn’t erase housing-related concerns, as median home prices fell for a sixth straight month.

But a growing feeling that Wall Street, which has had a big run-up since October, was due for a correction also played into Tuesday’s decline.

“I think that the market was prepared to pull back. The constellation of issues that were worrying the market came to a head,” said Quincy Krosby, chief investment strategist at The Hartford.

Just a week ago, the Dow had reached new closing and trading highs, rising as high as 12,795.92.

The broader Standard & Poor’s 500 index was down 50.33, or 3.47 percent, at 1,399.04, and the tech-dominated Nasdaq composite index was off 96.65, or 3.86 percent, at 2,407.87.

A suicide bomber attack on the main US military base in Afghanistan where Vice President Dick Cheney was visiting also rattled the market.

China’s stock market plummeted from record highs as investors took profits when concerns arose that the Chinese government may try to temper its ballooning economy by raising interest rates again or reducing more of the money available for lending.