A few generations ago, savers responded to financial panics with runs on banks, and even healthy institutions could fail if they could not raise enough cash quickly enough.

For a long time, that all seemed to be safely relegated to the past. But now the runs are back — and this time the targets are not banks but the securities that have replaced them as the prime generators of credit in the new financial system.

“Our current system of levered finance and its related structures may be critically flawed,” said William H. Gross, the chief investment officer of Pimco, a mutual fund company. “Nothing within it allows for the hedging of liquidity risk, and that is the problem at the moment.”

This problem has plagued the United States at regular intervals. The Panic of 1907 was halted only when the banker J. P. Morgan persuaded banks to stand together and halt the string of closings by lending money to threatened institutions. That led to the creation of the Federal Reserve, as Congress recoiled from the notion that the country’s financial health had relied on the wealth and wisdom of one private citizen.

Then the Depression, with a wave of bank failures, led to the establishment of deposit insurance. With that, savers became convinced that they need not worry about the health of their bank, and bank runs vanished.

But a new financial architecture emerged in the last decade — one that relied more on securities and less on banks as intermediaries. With the worth of those securities now being questioned — and no equivalent of deposit insurance — some who financed the securities want their money out, a fact that has created the 21st-century equivalent of a run on a bank.

Left to deal with the run are the institutions that were created to deal with the old system’s problems — notably the central banks like the Federal Reserve and the European Central Bank. But, in contrast to their close involvement with the banking system, these banks have little regulatory oversight of the securities that are in trouble and may not even know who is holding them.

At the heart of the new system was a decision to have loans financed directly by investors, rather than indirectly by bank depositors. Investors, ranging from hedge funds to wealthy individuals, had confidence in the arrangement because most of the securities were blessed as very safe by the bond rating agencies, like Moody’s and Standard & Poor’s.

The highly rated securities pay relatively low interest rates, but until now there were many willing to own them or to lend money to those who did own them. But there is no reason to hold them if there is any question about their safety — just as there was no reason to keep deposits in a bank that was facing a run amid rumors about its safety.

A result has been a freezing up of markets for many securities that, it turns out, were critical to the free flowing of credit. The problem first gained widespread attention when two hedge funds run by the brokerage firm Bear Stearns collapsed and a third Bear Stearns fund had to suspend redemptions as investors sought to get out even though there was no evidence that the fund was in trouble.

“The third Bear Stearns fund announcement was the key,” said Robert Barbera, the chief economist of ITG. “You have to believe that in the hedge fund and mutual fund complexes, there is a decision that is building that says, ‘I want to hold some Treasuries to have a cushion if I see redemptions.’ ”

The basis of the system was a belief that securities backed by bad credit could be very safe — so long as there were other securities that would suffer the first losses that came from defaults in pools of subprime mortgages or of loans to highly leveraged companies.

So far, none of those highly rated securities have failed to make their interest payments on time, but that fact is not enough to make anyone want to buy them. The rating agencies have downgraded some securities, and they are tightening their standards for new ratings.

Early this week, stock market investors around the world tried to reassure themselves that nothing was really wrong, and financial stocks bounced back after suffering sharp declines last week. Analysts argued that profits remained strong, as does world economic growth.

On Tuesday, the Fed declined to lower the federal funds rate, saying that despite financial market volatility and a decline in the housing market, “the economy seems likely to continue to expand at a moderate pace over coming quarters, supported by solid growth in employment and incomes and a robust global economy.”

But that comforting outlook did not help the credit markets recover, or persuade anyone to buy the newly questioned securities — at least at anything like the prices people had assumed. No one wants to sell the securities at very low prices — and in many cases they have borrowed heavily against them. So the markets have dried up.

Yesterday, BNP Paribas, a major French bank, said it could no longer value three investment funds that it managed, whose assets had been invested in highly rated securities that were backed by dubious mortgages.

“The complete evaporation of liquidity in certain market segments of the U.S. securitization market,” the French bank said, “has made it impossible to value certain assets fairly, regardless of their quality or credit rating.”

Adding to the problem is that the questionable securities are widely owned and sometimes have been repackaged to form the basis of other securities. European banks and funds own paper tied to subprime mortgages, and it is not clear who else does, or how investors will react.

Banks that are worried about their own liquidity decided this week to increase their reserves, which they can do by borrowing from other banks. Loans on such rates rose as a result of the added demand. Both the federal funds rate — the rate on loans of reserves between American banks — and the London Interbank Offered Rate leaped sharply yesterday.

The Fed — which conducts monetary policy by focusing on the fed funds rate — was forced to inject money into the system to bring the rate back down to its targeted level. And the E.C.B. lent almost 100 billion euros ($130 billion), to European banks.

If the current panic is just that — unreasoning fear — then such cash infusions may be able to let the new financial system weather the storm. Money can be lent to those owning the dubious securities, obviating the need to sell. As they eventually turn out to be good, the loans can be repaid and all will be happy.

On the other hand, if many of those securities turn out to be as bad as people now fear, some of those loans will not be good, and there may be more financial failures.

Yesterday, stock prices fell in Europe and kept declining in the United States, amid speculation over what other owners of the securities might surface as having problems. But American stock prices remain well above the levels they fell to in February, after a sudden drop in the Chinese stock market, and many stock investors still think all will work out acceptably.

The central banks, while clearly crucial to dealing with the loss of faith in the new financial system, lost influence under that system. Loans could be arranged by nonbanks, not subject to bank regulators, and the regulators were hesitant to impose rules that would not apply to all lenders. The lenders sold securities to finance mortgages that let people borrow at rates that — temporarily — were far lower than the Fed envisioned. That delayed the impact of the Fed’s attempts to raise interest rates in 2005 and 2006.

“That is important because it means the decline in the housing market is likely to continue,” Mr. Barbera said. If the American economy does continue to weaken, the Fed may feel forced to reduce interest rates sooner than it had expected, even if that move threatens to hurt the value of the dollar.

Prices in the futures market for federal funds show that just a few weeks ago investors thought there would be no Fed easing this year. Now they seem to think such a move is highly likely, and some expect it as early as next month.

But the Fed’s influence is limited when lenders are suddenly risk-averse. “The impetus of lowering interest rates may not help, if they don’t let you borrow in the first place,” said Kingman Penniman, the president of KDP Investment Advisors.

The new financial system is not the one the Fed was created to deal with, but it is the one it must try to handle.