Mr. Trichet also warned governments that they must attack the debt problem vigorously. “For us what is absolutely decisive is the commitment of governments of the euro area to take all measures needed to meet their fiscal targets this year and in the years ahead,” Mr. Trichet said in Basel.

The central bank, which had said buying bonds was not even on the agenda at its regular meeting last Thursday, announced the reversal early Monday after an emergency telephone conference by members of its Governing Council. The purchases began Monday, but the central bank did not immediately say which government bonds the banks are buying or what amounts. A Bundesbank spokesman also declined to provide details.

Michael Schubert, an analyst at Commerzbank in Frankfurt, said the central bank might initially buy Portuguese bonds as a signal that the Greece crisis will not be allowed to infect other euro zone countries. “This could be a successful means of putting a halt to the domino effect,” Mr. Schubert said in a research note.

The bank’s bond-buying program, which will be conducted via the central banks in nations that belong to the euro, could amount to about 110 billion euros, or about 5 percent of the total volume outstanding. The Fed has spent about the same amount in percentage terms buying bonds to stabilize American financial markets.

In its statement, the European Central Bank said that the liquidity that the bond purchases would pump into the European financial system would be “sterilized,” or offset with other monetary operations to drain liquidity from the system. In doing so, the bank seemed to be trying to answer criticism that buying bonds is the same as printing money and could lead to inflation.

According to Commerzbank, the expiration of 442 billion euros in one-year loans in June will automatically drain cash from the financial system.

However, the European Central Bank also announced new lending to prevent a repeat of late 2008, when banks’ doubts about each other’s solvency caused interbank lending to seize up.