WASHINGTON • Troubling new signs of a deep economic malaise touched off some of the worst stock market losses in history Wednesday, a day after the government announced a massive intervention that officials hoped would boost investor confidence.

New data showed that consumers stayed away from malls, nixed plans for new cars and made do with old clothes in September, forcing the largest monthly decline in retail sales in three years.

Federal Reserve Chairman Ben Bernanke added to the gloom, cautioning that the nation should not expect an economic rebound any time soon.

The Dow Jones industrial average fell 733.08 points, or 7.9 percent, its second-biggest point drop in history, while the Standard & Poor’s 500 index, a broader measure, sank 90.17 points, or 9 percent, the most since the crash of 1987, infamously dubbed Black Monday.

The market declines came after the Treasury Department said it would spend at least $250 billion to take ownership stakes in financial firms and insure most forms of bank debt. Officials had hoped those measures would calm investors’ nerves and heal the crippled financial system.

Bernanke said, “the turmoil in financial markets and the funding pressures on financial firms pose a significant threat to economic growth.” His remarks appeared to signal that the central bank was open to lowering its benchmark interest rate, which it cut just last week to 1.5 percent.

The credit crisis has penetrated so deeply into the American psyche that many consumers, whose spending is the most important component of economic activity, have become too scared to shop.

“The consumer has been hit over the head by so many two-by-fours that the consumer may end up going into a coma here,” said Brian Bethune, chief U.S. financial economist for consulting firm Global Insight. The rate banks charge each other for loans, a critical gauge of whether the government’s proposal is working, has barely shown any improvement since the Treasury’s new plan was unveiled. This rate, known as the London interbank offered rate, or Libor, remains higher than it was a week ago and about 61 percent higher than a month ago.

Joseph Stiglitz, a Nobel Prize-winning economics professor at Columbia University, said it was a “mystery” why Libor didn’t drop after the government guaranteed lending between banks.

Regulators pleaded for patience Wednesday, saying it would take some time for the effects of the government’s actions to work their way through the financial system.

“Stabilization of the financial markets is a critical first step, but even if they stabilize as we hope they will, broader economic recovery will not happen right away,” Bernanke said in a speech to the Economic Club of New York.

Bernanke also raised concerns that the banking industry has become overly consolidated as big financial institutions have collapsed or been swallowed up by one another during the crisis. Now the nation may have a “too-big-to-fail problem,” in which the collapse of any of the big banks could threaten the entire system.

Several key reports point to serious trouble ahead for the economy. Three of the nation’s largest banks reported that consumers are having increasing difficulty paying off their credit card debt and more are going into default.

Meanwhile, economic activity weakened across every region of the country last month as businesses and consumers pulled back, the Fed said in its monthly state of the economy, known as the “beige book.” Labor market conditions also deteriorated, though the Fed noted that inflation moderated in some areas.

The U.S. Commerce Department Wednesday released data that underscored how broadly shoppers have pulled back. Retail sales in September – which cover everything from sofas to sporting goods – dropped 1.2 percent compared with the previous month. Auto dealers were the biggest drag, with their sales falling 4.2 percent, followed by sales at furniture and clothing stores.

The auto industry was particularly hard hit last month. Compared with September 2007, sales at auto dealers were down 20 percent, the Commerce Department said.

The only two categories that posted gains last month were gas stations and health and personal-care stores, and even they rose less than half a percentage point. For the third quarter, consumer spending is expected to decline for the first time in 17 years.

New York University economics professor Nouriel Roubini, who had long been predicting a housing downturn that would trigger a recession, said he expected an economic slowdown to last 18 to 24 months. The average recession is about 10 months, he said.

In this Oct. 8, 2008 file photo, people look through the windows of the Nasdaq MarketSite Wednesday, Oct. 8, 2008 in New Unlike the last two relatively short recessions, the downturn we are entering could prove much longer and tougher. Photo by THE ASSOCIATED PRESS