But to the surprise of investors and analysts, the committee said it had decided to purchase an additional $750 billion worth of government-guaranteed mortgage-backed securities on top of the $500 billion that the Fed is already in the process of buying.

In addition, the Fed said it would buy up to $300 billion worth of longer-term Treasury securities over the next six months. That would tend to push down longer-term interest rates on all types of loans.

All these measures would come in addition to what has already been an unprecedented expansion of lending by the Fed. The central bank also said it would probably expand the scope of a new program to finance consumer and business lending, which gets under way this week.

In effect, the central bank has been lending money to a wider and wider array of borrowers, and it has financed that lending by using its authority to create new money at will.

Since last September, the Fed’s lending programs have roughly doubled the size of its balance sheet, to about $1.8 trillion, from $900 billion. The actions announced on Wednesday are likely to expand that to well over $3 trillion over the next year.

Despite a trickle of encouraging data in the last few weeks, Fed officials were clearly still worried and in no mood to cut back on their emergency efforts.

Fed policy makers sharply reduced their economic forecasts in January, predicting that the economy would continue to experience steep contractions for the first half of 2009, that unemployment could approach 9 percent by the end of the year and that there was at least a small risk of a drop in consumer prices like those that Japan experienced for nearly a decade.