Democrats took control of the US Congress on Friday, promising ambitious reforms and tough new oversight of Republican President George W Bush's policy in Iraq.
In his official capacity as president of the US Senate, Vice President Dick Cheney swore in 33 new and returning members of the 110th Senate in an official midday ceremony, as the lawmakers pledged "to well and faithfully discharge" their legislative duties.
In the House, top Democrat Nancy Pelosi, 66, became the first woman to take the job of speaker.
The chamber's other 434 members were to be sworn in at a separate House ceremony.
"In this House, we may belong to different parties, but we serve one country. We stand united in our pride and prayers for our men and women in the armed forces.
They are working together to protect America, and we, in this House, must also work together to build a future worthy of their sacrifice," she said.
The oath-taking formalizes the sweeping new political order in Washington in the aftermath of November's historic legislative elections, and signals dramatic new political change as Congress's most conservative political caciques cede power.
As of today, Holocaust survivor Tom Lantos, 78, who voted for the Iraq war but now is calling for dialogue with Syria and Iran, will head the House International Relations Committee.
In the Senate, another fierce White House opponent, Harry Reid, 67, will be the new majority leader, while Joseph Biden, 63, who hopes to run for the White House in 2008, will head the Senate Foreign Relations Committee.
Reid vowed at a press conference on Thursday that a "new day" had dawned in Washington, and he pledged to work cooperatively with 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," he said after a meeting with his Republican counterpart, Mitch McConnell.
McConnell, now in the minority party, agreed that it was high time to "get past the level of partisanship that we've witnessed in recent years and develop stronger personal relationships, as well as work across party aisles."
On the margins of the formal events, former president Bill Clinton told reporters he expects a new spirit of bipartisan cooperation in Congress as Democrats take control of the usually fractious US legislature.
"A lot of new people are coming in, and I think it will be good," said Clinton, whose wife, New York Senator Hillary Clinton, was reelected in November.
"I'll be surprised if you don't see a lot of interesting, bipartisan positive things happening over the next six months," he told reporters in impromptu remarks just before the swearing-in ceremony.
After the official oath-taking ceremony in the House, newly elected Muslim US Representative Keith Ellison was to be sworn during a private ceremony on a copy of the Koran once owned by president Thomas Jefferson.
Speaking to ABC News in a broadcast which aired today, Ellison, the first Muslim to be elected to the US Congress, said it was only natural that he would take his oath on the Koran, the book of his Islamic faith.
"It's the scripture that I read every day, and it's the book that I draw inspiration from," said the incoming lawmaker, who converted from Roman Catholicism to Islam while a student in college.