Democrats have taken control of the US Congress after 12 years in opposition.
The first day of the new Congress was dominated by the swearing-in of the 535 House and Senate members.
California Democrat Nancy Pelosi was elected speaker of the House of Representatives with 223 votes, a majority of the 435-member House.
The 66-year-old becomes the first woman to hold the office and will be the second in line to the presidency, after the vice-president.
Before she was sworn in, Ms Pelosi said she accepted the position in the 'spirit of partnership, not partisanship' and said she looked forward to working 'for the good of the American people'.
She is among the most liberal of Democrats thanks to her pro-choice, anti-war stance, and is credited with uniting her party after years of opposition.
Having swept US President George W Bush's Republicans from power in 7 November elections, Democrats have promised to push for a phased withdrawal of US troops from Iraq and clean up how the scandal-rocked Congress operates.
Republicans are expected to be challenged on the issue of expanding stem cell medical research, a measure President Bush vetoed last year.
Democrats also say they will take on the powerful US oil companies by proposing an end to some federal subsidies and investing the savings in renewable energy.
While these initiatives are expected to move quickly through the House, some of them could face tougher going in the Senate despite the confirmation of fierce White House opponent Harry Reid, as majority leader.
