The country’s four biggest banks announced yesterday that they had each borrowed $500 million from the Federal Reserve, taking an unusual step to ease the credit squeeze that has been rattling the financial system for weeks.

The banks — Citigroup, Bank of America, JPMorgan and Wachovia — said that they had tapped the so-called discount window of the Federal Reserve Bank of New York, five days after the central bank lowered the rate and loosened its collateral standards in an effort to inject more money into the credit markets.

The coordinated moves were seen as largely symbolic, aimed at removing the stigma of borrowing from the discount window, which is regarded as a last resort for financial institutions. All four banks can borrow money more cheaply elsewhere, and all said they had “substantial liquidity.”

In a conference call on Friday, Fed officials prodded the banks to take the action, according to people briefed on the situation.

The Federal Reserve said at the time that using the discount window should be viewed as a sign of strength, not weakness.

“I think it is a show of solidarity with the Fed’s objective with removing any taint of borrowing from the discount window,” said Paul McCulley, a portfolio manager at Pimco, a big fixed-income money management firm. “There was no obvious need for any of those institutions to borrow.”

Nonetheless, the move puzzled longtime market watchers, some of whom are expecting bigger troubles.

“Going to the discount window is like someone on the Upper East Side being seen in a Wal-Mart,” said Charles R. Geisst, a financial historian at Manhattan College. “The T-shirts may be cheap, but why would you?”

He added: “The banks are circling the wagons. Somebody’s got a problem.”

Other big banks have also tapped central banks for financing. On Tuesday, Barclays, a British bank, borrowed £314 million ($622 million) from the Bank of England’s standing facility, which is used to square accounts between banks. Deutsche Bank confirmed that it had borrowed from the Federal Reserve’s discount window on Friday, though it did not formally publicize the action until yesterday.

Wells Fargo, the nation’s fifth-largest bank, declined to comment about whether it had borrowed from the discount window, but said it had “ample funds.”

It is unclear when any of the four big banks used the discount window, or if any other banks had borrowed money. The Federal Reserve does not report data until this evening, but those are aggregate weekly numbers and do not name the borrowers.

Banks have long been reluctant to borrow from the discount window, because the disclosure that capital was being sought there was akin to announcing a financial crisis, which could alarm investors and make the situation worse.

By encouraging the largest banks to use the discount window and promoting it as a sign of strength, the Federal Reserve was trying to change investors’ perceptions.

“If you have a pipe to get liquidity in the system but nobody will use it, that limits its effectiveness,” Mr. McCulley said.

He added, “It was effectively an act of statesmanship by the big four banks,” noting that $500 million is a small amount for those financial institutions.

He said those banks could borrow federal funds at rates that are typically less than the discount window borrowing rate, which was lowered half a percentage point, to 5.75 percent, on Friday.

The borrowing announcements helped bolster investor confidence and lift the stock market for the second consecutive day. But some analysts said the main purpose was to send a message to the market for commercial paper and the broader bond market, both of which have been rattled since at least mid-July because of difficulties in the subprime mortgage market.

“From the Fed’s point of view, the risk was that banks would stop lending. That immediately pushes the pressure of the credit markets right into the U.S. economy,” said Brad Hintz, a Sanford C. Bernstein & Company analyst who was once chief financial officer for Lehman Brothers.

“By opening up the window, the Fed is really saying: ‘You go out and lend because we will lend to you,’ ” he added. “This tells us that confidence is coming back to the fixed-income market.”

In a joint statement, JPMorgan, Bank of America and Wachovia said that they were using the discount window in an effort to “encourage its use by other financial institutions.” Citigroup said separately that it was “pleased to inject liquidity into the financial system” and would “stand ready to continue to access the discount window as clients and needs warrant.”

Announcing its action yesterday, Deutsche Bank said that its decision to borrow on Friday was taken in support of the Fed’s efforts and the overall market.