"I accept this gavel in the spirit of partnership, not partisanship, and look forward to working with you on behalf of the American people," Pelosi said. "In this House, we may belong to different parties, but we serve one country."
Both Democrats and Republicans alike pledged cooperation despite years of bitter partisanship and gridlock, to try to get the new Congress off on a productive note.
House Democrats also were ready to impose a ban on gifts from lobbyists and a clampdown on travel funded by private interests — measures crafted in response to the ethics scandals that weakened Republicans in last fall's elections.
Pelosi will lead a fractious House divided 233-202, with Democrats claiming control for the first time since 1994.
"The election of 2006 was a call to change — not merely to change the control of Congress, but for a new direction for our country," Pelosi said in remarks prepared for delivery after her swearing in. "Nowhere were the American people more clear about the need for a new direction than in Iraq. The American people rejected an open-ended obligation to a war without end."
Democrats maintain a tenuous hold on a Senate divided 51-49, with ailing South Dakota Democrat Tim Johnson slowly recovering in a Washington hospital weeks after suffering a brain hemorrhage.
The fragile Senate margin ensures little Democratic-sponsored legislation can pass without support from at least some Republicans.
"Our efforts are going to be to work in a bipartisan basis in an open fashion to solve the problems of the American people," said new Senate Majority Leader Harry Reid, D-Nev.
Ten new senators and more than 50 new members of the House of Representatives, most of them Democrats, were being sworn in, CBS News correspondent Sharyl Attkisson reports.
Among those taking the oath of office was Joe Lieberman, who returned to the Senate for a fourth six-year term after losing a raucous Democratic primary in the northeastern state of Connecticut but winning in November running as an independent.
The day capped the rise of several Democratic veterans to powerful committee posts — including Charles Rangel of New York as chairman of the tax-writing Ways and Means Committee and David Obey on the powerful Appropriations panel — after 12 dispiriting years in the minority.
House Republicans, meanwhile, adjusted to their unaccustomed roles out of power, grousing about being shut out of any chance to affect the early agenda.
"This is a missed opportunity to really change the way that the House does business. I think the average American wants to believe that their House member has as much say as any of the other 435 members," said Rep. Adam Putnam, R-Fla.
House Democrats planned quick action on legislative priorities that included boosting both the minimum wage and stem cell research, as well as stiffer ethics rules. Democrats also said they would pressure President Bush to bring the troops home from Iraq.
"They have a very ambitious schedule of things to do. They are going to ban all travel on corporate airplanes. They are going to make congressmen who slip in secret funding requests, what we call 'earmarks', identify themselves. It's ambitious but on the other hand there are a lot of loopholes," CBS News chief Washington correspondent Bob Schieffer said.
Reid, a soft-spoken but tough inside player — took the reins of the notoriously unwieldy Senate, hosting both Democrats and Republicans at a rare closed-door conference Thursday morning in the Old Senate Chamber. The aim was to set a bipartisan mood after years of political rancor.
"Our efforts are going to be to work in a bipartisan basis in an open fashion to solve the problems of the American people," Reid said afterward.
The meeting, said top Senate Republican Mitch McConnell of Kentucky, gave senators in both parties "a chance to express some of their quiet frustrations that we get past the level of partisanship that we've witnessed in recent years."
The Democratic-led Congress also opened a new chapter in the presidency of Mr. Bush, who faces divided government as he cements his legacy in his final two years in the White House. Mr. Bush had a light public schedule Thursday, intended at least in part to let the new Congress have its day.
Anti-abortion protesters greeted Pelosi, D-Calif., as she began the day at a prayer service at a Catholic church on Capitol Hill before being sworn in as speaker in the afternoon by Rep. John Dingell, D-Mich., the longest serving member of the House. Pelosi then was to address the House.
Dingell administered the same oath to former Speaker Newt Gingrich, R-Ga., 12 years ago when Republicans seized the House after 40 years of Democratic control — and he's set to get back his gavel as the chairman of the House Energy and Commerce Committee.
Vice President Dick Cheney swore in the new and returning senators, beginning with a group including Senate President Pro Tem, Robert C. Byrd, D-W.Va. — third in the line of presidential succession — elected for a record ninth term. In the gallery overhead, former President Clinton and daughter Chelsea applauded and waved to Sen. Hillary Rodham Clinton of New York, who was sworn in for a second term.