Republicans take control of US House of Representatives
Republicans have formally taken control of the US House of Representatives, pledging to cut government spending and repeal President Obama's health reform.
New House Speaker John Boehner struck a humble tone in his opening address, but analysts foresee two years of fierce political battle ahead.
"The people voted to end business as usual," said Mr Boehner of Ohio.
With Democrats controlling the Senate and White House, Republicans stand little chance of enacting their agenda.
The Republican Party took power in the House on Wednesday after winning a majority in the November mid-term elections, thanks in part to the anti-government Tea Party movement.
The party had lost control of the House in 2006 amid Republican President George W Bush's fading popularity with many voters.
Mr Boehner, a staunch conservative and the son of a bar owner, was sworn in after spending the past two years as one of Mr Obama's fiercest opponents in Washington.
At the opening of the 112th Congress, outgoing Speaker Nancy Pelosi, a liberal San Francisco Democrat, passed him the speaker's gavel, calling the transition a "strong symbol of peaceful democracy".
In his opening remarks, Mr Boehner said the objective of Republicans was to give government back to citizens of the US and provide honesty and accountability.
"No longer can we fall short, no longer can we kick the can down the road," he said. "The people voted to end business as usual, and today we begin to carry out their instructions."
Meanwhile, in the Senate, Democratic Leader Harry Reid seemed to acknowledge upcoming Republican challenges, and spoke of the need for reform to ensure legislation is not blocked in the chamber.
"We have to do even more to help middle-class families, to create jobs, to hasten our energy independence, to improve our children's education and to fix our broken immigration system," he said.
The BBC's Paul Adams, in Washington, says there are tough fights ahead as the president, determined to press ahead with his reform agenda, locks horns with a Republican Party emboldened by its successes in November.
Need for compromise
Republican leaders have vowed to slash spending by as much as $100bn, scrap "job-killing" government regulations, crack down on undocumented immigration, cut diplomatic and foreign aid funds, and investigate the administration.
But the Democrats retain control of the US Senate, and Mr Obama wields the presidential veto pen, so any legislation passed in the next two years will have to be the product of careful compromise and deal-making, analysts say.
The Republicans' opening move, however, is far from conciliatory. Next week, they will hold a purely symbolic vote to repeal Mr Obama and the Democrats' healthcare reform law, their signature achievement of the past two years.
The move is expected to pass in the House, but fail in the Senate, but will be followed by a protracted attempt to pick the reform to pieces, our correspondent says.
Add to this a series of bitter debates over spending and how to control the country's budget deficits, and the scene is set for a tempestuous political season, our correspondent adds.
On Thursday, the Republicans will have the US Constitution read aloud in the House chamber as it gets down to business, a gesture in line with many conservatives' view that Democrats have overstepped their constitutional authority in passing sweeping regulations.