The Federal Reserve, saying for the first time that the recent disorder in the financial markets has raised the risk of an economic downturn, took the unusual step yesterday of encouraging the nation’s banks to borrow directly from the Fed, particularly to support home mortgage lending.

The early morning announcement of a drop in the so-called discount rate, made even before the stock market opened, sent Wall Street soaring as relieved investors celebrated the Fed’s tacit acknowledgment that it had made a mistake last week in playing down concerns over the spreading distress in the credit markets. After falling for six straight days, the Dow Jones industrial average shot up 233.30 points, to 13,079.08, a 1.8 percent increase.

The Fed, while not yet cutting a rate that wields more influence over the economy, moved to stimulate lending in part because it recognized that even well-to-do families with good credit ratings were having difficulty getting mortgages. That problem, radiating through the housing market and then the broader economy, had “the potential to restrain economic growth,” the Fed said.

The sudden action also suggested that the nation’s central bankers were worried that a major financial institution might be at risk of failure if the Fed did not move before its policy makers gather for their next regular meeting, on Sept. 18.

Analysts predicted that the Fed was likely to go further, cutting market interest rates soon, perhaps even before its meeting next month if its action yesterday does not help to shore up confidence on Wall Street and elsewhere in the economy.

The mechanism for lifting the mortgage market was a cut of half a percentage point, to 5.75 percent, on borrowing from the discount window, a normally rare practice in which the Fed lends directly to banks facing some sort of trouble.

“We want to make it very clear that we are happy to lend,” one Fed policy maker said, “and we are eager for banks to use the discount window.”

In a conference call yesterday, Fed officials told major banks that discount window borrowing would be viewed as a sign of strength, not weakness.

Just the way in which the Fed acted showed how quickly its previous concerns about the greater risk of inflation had evaporated. The Fed’s standard practice, for more than a decade, has been to change rates and issue statements only at scheduled meetings of policy makers. Inter-meeting actions only underscore the dire nature of the situation.

But as stock prices moved wildly once again on Thursday, the Fed’s chairman, Ben S. Bernanke, convened a 6 p.m. teleconference call, and after an hour of discussion, the 10 policy makers present voted unanimously for the discount rate cut and for an assessment of the economy clearly more negative than the upbeat one issued just 10 days earlier.

That earlier statement, issued after the Fed’s Aug. 7 meeting, said that the economy “seems likely to continue to expand at a moderate pace over coming quarters.” The new description repeated the view that the economy would expand at a “moderate pace,” but added “that the downside risks to growth have increased appreciably.”

William Poole, president of the Federal Reserve Bank of St. Louis, was missing from the conference call and Fed officials explained carefully to reporters that he was absent only because of a conflict with a long-scheduled dinner appearance, adding that suddenly canceling that appearance might alarm the marketplace. Mr. Poole has publicly opposed a cut in the federal funds rate.

Indeed, no change was made yesterday in the more important federal funds rate, which the Fed uses as its main policy tool, cutting it to stimulate a flagging economy or raising it to rein in a potentially inflationary one. Still, yesterday’s action suggested that the funds rate itself — which directly affects what businesses pays for loans and what ordinary Americans pay for home equity borrowing, car loans and various other consumer credit — would soon be cut.

“What the Fed is really saying is that this recovery is more in danger than it has been since it started nearly six years ago,” said Jan Hatzius, chief United States economist for Goldman Sachs, who predicted an initial cut in the funds rate to 5 percent, from the present 5.25 percent.

The discount move, coupled with formal acknowledgment that the economy might be in trouble, represents Mr. Bernanke’s boldest attempts to relieve the financial crisis, his first real test since becoming Fed chairman 18 months ago.

The Fed’s admission that the economy might not be so healthy suggested to Mr. Hatzius and others on Wall Street that an initial cut in the federal funds rate, which has been at 5.25 percent for nearly 14 months, could be followed by several more in the months ahead as the Fed braces for the possibility of a weakening economy, dragged down by a shrinking housing market.

“This is a watershed statement from the Fed to let the markets and politicians know that the Fed is prepared to act as needed,” Stuart G. Hoffman, chief economist at PNC Financial Services Group, wrote in a note to clients.

Fed policy makers and Treasury officials said that in cutting the discount rate, the Fed’s principal goal was to shore up the market for creditworthy mortgages, including those for more expensive homes.

While that market is primarily for more affluent borrowers, Fed officials said they were worried that average Americans would suffer, too, if the troubles in the so-called jumbo mortgage market infected the rest of the economy. The collapse of so many subprime mortgage securities had spread to the larger market, and lenders of high-quality mortgages were having trouble selling the loans they made.

So the Fed said to the banks in effect: borrow at the discount window, using the mortgages as collateral, at a rate of 5.75 percent, down from 6.25 percent. Moreover, banks were encouraged, if necessary, to roll over the loans every 30 days, an easing of the usual 24-hour rule.

That brought the discount window into the financial crisis, which the Fed, until yesterday, had dealt with in a different manner. It did so by adding billions of dollars to the banking system, to keep the federal funds rate at 5.25 percent. This is the interest that banks and other lending institutions pay to borrow from each other to finance their operations.

In the midst of the crisis, the federal funds rate spiked to a level above 6 percent.

Cutting it will inevitably raise the charge that the Fed is bailing out hedge funds and others who borrowed billions of dollars to invest in subprime mortgages, ignoring the risk that many of the low-income households who got these mortgages would default.

Lower interest rates raise the odds that risk takers can sell bad investments with less of a loss.

“You would be mitigating their problems,” said Alan Blinder, a Princeton University economist and a former Fed governor, but “I would count that as a social cost of saving the economy.”

What remains unclear is how much damage, if any, the economy has suffered from the market turmoil. The Fed promised “to act as needed to mitigate the adverse effects,” but data documenting any damage takes weeks to collect.

Home construction is clearly down, and recent economic reports show that consumption has weakened since the first quarter.

“If there was evidence of a significant decline in spending, that would induce the Fed to cut rates, but there is no evidence of a significant decline in spending,” said Mark Gertler, a New York University economist who did research with Mr. Bernanke when the Fed chairman was a Princeton professor.

The earliest damage would show up in consumer sentiment. The University of Michigan’s Survey of Consumers released a poll yesterday showing that Americans were somewhat more pessimistic because of costly food and fuel, not the unfolding financial crisis. “People have not yet begun to calculate how the market turmoil might affect their own situation,” said Richard T. Curtain, director of the Michigan survey. “That is still in the future, if it ever happens.”