Analysts said the moves would provide only a slight economic benefit.
The action the Fed announced Wednesday is modest compared with previous steps it's taken. The Fed won't expand its $2.9 trillion holdings; it's just rebalancing them.
It will sell $400 billion of its shorter-term Treasurys to buy longer-term Treasurys by June 2012. And it will reinvest principal payments from its mortgage-backed securities, to help keep mortgage rates at super-low levels.
Fed policymakers announced the moves after a two-day meeting. Three members out of 10 dissented from the decision. The Fed acted despite criticism from Republicans who have warned that such steps could ignite inflation.
"The actions the Fed has taken are helpful," says Josh Feinman, global chief economist at DB Advisors. "They will help hold down long-term rates, but they're no panacea."
The Fed left open the possibility of taking further action to try to strengthen the economy.
Stocks dropped immediately after the announcement at 2:15 p.m. and then continued falling. The Dow Jones industrial average closed down about 283 points. The yield on the 10-year Treasury note tumbled, and its price rose.
The plan the Fed unveiled Wednesday, dubbed "Operation Twist," resembles a program the Fed used in the early 1960s to "twist" long-term rates lower relative to short-term rates.
In its statement, the Fed noted that the economy is growing slowly, unemployment is high and housing remains in a prolonged slump.
Analysts say the shift in the Fed's portfolio could reduce borrowing costs and perhaps raise stock prices.
"This is a measured response to weak economic conditions," said David Jones, head of DMJ Advisors and the author of four books on the Fed. "The Fed is still trying but it can only do so much."
In June, the Fed completed a $600 billion bond-buying program that many economists have credited with keeping rates low.
Once the Fed announced last month that it would expand its September policy meeting from one to two days, economists anticipated some new action. Chairman Ben Bernanke had said the Fed was considering a range of options.
The Fed's move Wednesday came despite a rift within the central bank. The three members who dissented also did so at the Fed's August meeting - the most negative votes in nearly two decades.
The three dissenters - Richard Fisher, Narayana Kocherlakota and Charles Plosser, all regional Fed bank presidents - have said the Fed's policies may be raising the risk of high inflation. They favor giving the economy more time to heal without further Fed action.
Still, the central bank is under pressure to revive an economy that has limped along for more than two years since the recession officially ended.
In the first six months of this year, the economy grew at an annual rate of just 0.7 percent. The housing market remains depressed. The unemployment rate is 9.1 percent. In August, the economy didn't add any jobs, and consumers didn't increase their spending on retail goods.
Most economists foresee growth of less than 2 percent for the entire year. They say the odds of another recession are about one in three.
The Fed has offered its own bleak outlook. At its August policy meeting, it said the economy would likely struggle for at least two more years. As a result, it said it planned to keep short-term rates near record lows until mid-2013, as long as the economy remained weak.
Historically low mortgage rates, now averaging of 4.09 percent on a 30-year fixed loan, have done little to boost either home purchases or refinancings. Many would-be buyers don't have the required down payments or are reluctant to buy at a time when prices are still falling in many areas.
And many homeowners have no equity and can't refinance.
Bernanke's policymaking has incited criticism from congressional Republicans and GOP presidential candidates. Some have argued that the Fed's $600 billion bond-buying program, which ended in June, raised inflation pressures, weakened the dollar's value against other currencies and contributed to a spike in oil prices.
On Monday, the four highest-ranking Republicans in Congress sent Bernanke a letter cautioning the Fed against taking further steps to lower interest rates. Their letter suggested that lower rates could escalate the risk of high inflation.
Texas Gov. Rick Perry, who is seeking the GOP nomination for president, has gone so far as to say Bernanke would be "almost treasonous" to launch more bond buying.
The Fed's efforts to stimulate the economy through low rates are occurring at a time when Congress is focused more on shrinking spending.
President Barack Obama has proposed a $447 billion job-creation program made up mainly of tax cuts and public works spending. Obama also wants the richest Americans to pay higher taxes to help cut federal budget deficits.
Obama's proposals face an uncertain fate in Congress. Republicans have dismissed his deficit-reduction plan and have focused on efforts to reduce spending and keep taxes low for everyone.