Inauguration: Barack Obama is sworn in for second term
Barack Obama has taken the oath of office in Washington DC to inaugurate his second term as US president, before hundreds of thousands of spectators.
Mr Obama, 51, who is the 44th president of the US, was sworn in by Chief Justice John Roberts.
Monday's inaugural festivities in the US capital include black-tie balls, a parade and musical performances.
In his inaugural address, the Democratic president is now laying out his vision for the next four years.
He was formally sworn in at a small White House ceremony on Sunday - the US Constitution dictates presidential terms begin on 20 January.
In the famous Blue Room with his wife and daughters, the 44th president rested his hand on a Bible that was used for many years by Michelle Obama's family, as Justice Roberts administered the oath of office.
According to the words prescribed by the US Constitution, Mr Obama swore he would "faithfully execute the office of president of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States".
He repeated those words at Monday's public inauguration, placing his left hand on Bibles owned by legendary American civil rights leader Martin Luther King Jr and President Abraham Lincoln.
Former Presidents Bill Clinton and Jimmy Carter, along with dozens of senators, congressional leaders and other dignitaries have attended the event at the US Capitol.
In his inaugural address, Mr Obama is not expected to preview his policy agenda on Monday. He will address that in next month's State of the Union speech, asking bitterly divided lawmakers to come together and reduce gun violence, overhaul the immigration system and reduce the yawning federal budget deficit.
Speaking at a reception for supporters late on Sunday, Mr Obama suggested he would dwell on the "common good" and the "goodness, the resilience, neighbourliness, the patriotism" of Americans.
"What we are celebrating is not the election or the swearing-in of the president," Mr Obama said. "What we are doing is celebrating each other and celebrating this incredible nation that we call home."
By Monday afternoon, Mr Obama will have taken the oath four times - as many as President Franklin D Roosevelt.
Four years ago, Mr Obama repeated the oath privately to make sure all constitutional obligations were met after Justice Roberts tripped over the words in the public ceremony.
Following Monday's ceremony, Mr Obama will have the traditional lunch with US lawmakers in the capitol building's Statuary Hall.
Later, the president is expected to walk at least part of the parade route down Pennsylvania Avenue toward the White House.
Vice-President Joe Biden, who was also sworn in on Sunday - by Supreme Court Justice Sonia Sotomayor - repeated his oath publicly as well.
In the evening, the Obamas will don formal evening attire for several lavish inaugural balls in Washington.
The zone surrounding the National Mall in the US capital is in virtual lockdown, with movement of people and vehicles tightly restricted.
White tents, trailers and generators are set up along the parade route, while nearby buildings have been adorned with red, white and blue bunting.
Officials estimate about 700,000 people were to attend the inauguration, down significantly from 2009, when about 1.8 million people witnessed Mr Obama be sworn in as America's first black president.
About 260,000 people attended George W Bush's second inauguration ceremony in 2005, with an additional 150,000 lining the parade route, officials said.
"We wanted to see history, I think, and also for the children to witness that anything is possible through hard work," David Richardson of Atlanta, in the crowd with his two young children, told the Associated Press.