A still-popular Barack Obama has taken the presidential oath of office for a second term, facing a troubled future but hoping to leave behind a battering four years at the helm of a government mired in ugly political division.
When Mr Obama first took office as the 44th US president, many Americans hoped the symbolism of the first black man in the White House was a turning point in the country's deeply troubled racial history.
Mr Obama vowed to moderate the partisan anger engulfing the country, but the nation is only more divided four years later, perhaps as deeply as at any time since the US Civil War 150 years ago.
Mr Obama was sworn in by Supreme Court Chief Justice John Roberts during a brief ceremony with his family in the White House Blue Room, meeting the legal requirement that presidents officially take office on January. 20. Because that date fell on a Sunday this year, the traditional ceremonies surrounding the start of a president's term were put off to Monday, which coincides this year with the birthday of revered civil rights leader Martin Luther King. He was assassinated in 1968.
Mr Obama made no special remarks at Sunday's ceremony, surrounded by portraits of former White House residents. "I did it," he said.
On Monday, he will repeat the oath and give his inaugural speech on the steps of the US Capitol before hundreds of thousands of people. He then makes the traditional journey, part of it on foot, down Pennsylvania Avenue to the White House. Fancy dress balls, fewer than in 2009, consume the evening hours.
While Mr Obama convincingly won a second term, the jubilation that surrounded him four years ago is subdued this time around - a reality for second-term presidents.
He guided the country through many crushing challenges after taking office in 2009: ending the Iraq war, putting the Afghan war on a course toward US withdrawal and saving the collapsing economy. He won approval for a sweeping health care overhaul. Yet onerous problems remain, and his success in resolving them will define his place in history.
He faces bitter confrontation with opposition Republicans over gun control, avoiding a default on the nation's debts, cutting the spiralling federal deficit and preventing Iran from building a nuclear weapon.
Joe Biden was sworn in for his second term as vice president earlier on Sunday, taking the oath from Supreme Court Justice Sonya Sotomayor at his official residence at the US Naval Observatory.