US President Barack Obama has taken the oath of office to begin a second term threatened by strife at home and abroad and amid inaugural rituals lacking the hope and historic promise of 2009.
Obama, with a slight smile, took the oath at an intimate, private ceremony in the Blue Room of the White House lasting less than a minute, raising his right arm and placing his left hand on a family Bible.
Obama sworn in for second term
Chile investigates mass dolphin deaths
Samsung soars with AUS$9.3bln earnings
Huge dinosaur track found in Bolivia
Fatal floods wreak havoc in Nepal
Indonesian executions ahead of schedule
Pope takes a tumble
Pope Francis: Terror is part of a wider war
Obama sworn in for second term
RAW VISION: US President Barack Obama takes the oath of office in an intimate White House ceremony.
The president solemnly swore to "faithfully execute the office of President of the United States" and to "preserve, protect and defend the Constitution of the United States, so help me God".
Chief Justice John Roberts, who stumbled when swearing in Obama to open his first term in 2009, read each line of the oath out loud, before the president repeated words first intoned by George Washington 224 years ago.
After the ceremony, Obama hugged his wife, Michelle, and children Malia, 14, and Sasha, 11, before quipping to his younger daughter: "I did it".
Obama will take the oath of office for a second time at a time-honoured outdoor public ceremony at the US Capitol on Monday (Tuesday AEDT).
Vice-President Joe Biden was sworn in before his boss at an early morning ceremony at his official residence, before the two laid a wreath at the Tomb of the Unknowns at Arlington National Ceremony.
Supreme Court Justice Sonia Sotomayor - picked by Obama to be the first Hispanic judge to sit on the top court in his first term - made her own slice of history by leading Biden as he took the oath.
As his first term waned, Obama worshipped at a prominent African American church in Washington, where the Reverend Ronald Braxton adapted the "Forward" motto of the president's re-election campaign to the story of Moses.
"Forward was the only option," said Braxton.
Obama, 51, will embark on a second term at a time of deep partisan division in Washington, and will face foreign crises testing his legacy, including Iran's nuclear program and resurgent Islamist militancy in North Africa.
Senior aide David Plouffe said the president would use his second inaugural address before an expected 500,000 plus crowd - much smaller than in 2009 - to stress the national truths Americans share.
"He is going to talk about how our founding principles and values can still guide us in today's modern and changing world," Plouffe said on the ABC News show, This Week.
"He is going to say that our political system does not require us to resolve all of our differences or settle all of our disputes, but it is absolutely imperative that our leaders try and seek common ground."
Obama was taking the oath of office on Sunday to comply with the US Constitution, which dictates his first term ends at noon on January 20.
Tradition states that when that date falls on a Sunday, a private swearing-in is followed on Monday by the public festivities, including the second oath-taking, the address, parade and glittering inaugural balls.
Obama knows that for second-term presidents, power quickly wanes and political potholes await.
The second-term "curse" often strikes: Richard Nixon resigned, Bill Clinton was impeached, George W Bush's image was shattered by Iraq and Hurricane Katrina and Ronald Reagan's legacy was marred by the Iran-Contra scandal.
Plouffe said, however, that with a full agenda, including immigration reform, gun control and speeding the economic recovery, Obama would hardly be shuffling around the White House looking for things to do.