"Every American that participated in this election, whether you held a Romney or Obama sign - you made your voice heard.
"A decade of war is ending. A long campaign is over. And whether I earned your vote or not, I have listened from you, learned from you, and [that] made me a better President.
"I return more determined and inspired than ever about the work there is to do. Tonight you voted for action, not politics as usual.
"Thank you for believing all the way, over every hill and through every valley you lifted me up and I will always be grateful."
He spoke of his wife, Michelle, saying: "I would not be the man I am today if it were not for the woman I married 20 years ago. Michelle, I have never loved you more, never [been] prouder."
And he spoke of his defeated rival, Mr Romney, saying they had "battled fiercely but it is only because we love this country deeply".
"In the weeks ahead I look forward to sitting down with [Mr Romney], to talk about how we can work together to take this country forward."
Mr Romney earlier conceded defeat in Boston, telling supporters he had called Mr Obama to congratulate him on his victory, and that he would pray "the President will be successful in guiding our nation".
But Mr Obama faces a bitterly divided country where the popular vote was split almost equally between the President and his Republican challenger.
The result was also unlikely to end the prevailing gridlock in a divided Congress whose inaction threatens the timid economic recovery.
At precisely 11.19pm New York time, the top of the Empire State Building turned blue for Mr Obama as CNN projected the re-election of the country's first black President by winning more than 270 electoral college votes. Crowds of his jubilant supporters across America burst into spontaneous clapping and cheering on city streets.
Heeding the candidates' warning that a big turnout was needed for this "choice of a lifetime", voters defied those who had spoken of diminishing enthusiasm for Mr Obama, whose message of hope and change in 2008 mobilised black and Hispanic minorities, and young people.
In some swing states such as Iowa, Florida and Virginia, which were all projected to be won by Mr Obama in close races, the turnout was higher than in 2008. The African American turnout in Pennsylvania exceeded that of four years ago, according to CNN. One by one, battleground states began to fall into the President's lap, depriving Mr Romney of pathways to the White House. An early one was Wisconsin, the home state of Mr Romney's vice-presidential running mate, Paul Ryan.
Mr Romney lost his home state of Massachusetts and New Hampshire where he had begun and ended his second bid for the presidency. Votes in the crucial swing state of Ohio were still being counted, but that state too was projected to go to Mr Obama.
The breakdown of voter habits reflects the demographic makeup of both candidates' supporters at their campaign rallies. Mr Obama's were racially mixed and from all walks of life while Mr Romney's were mainly white and middle class.
Mr Romney from the outset had struggled to overcome the perception that he was a super-rich businessman concerned only with the interests of the wealthy. Although a majority of voters said they trusted his business expertise, they ultimately rejected him at a time when the economy has been steadily and gradually growing in recent months.
He also seems to have paid the price for taking a hard line on immigration during the primary debates of the campaign, the most costly ever. Taking into account the congressional races as well as the presidential contest, outside groups, corporations and unions spent a total US$6 billion ($7.2 billion) in support of candidates, which mainly went on negative television ads.
But the critical factor in Mr Romney's defeat was the "October surprise" of Hurricane Sandy, which struck the east coast 10 days ago.
Mr Obama received high marks from the public for his effective handling of the storm and for reaching across the aisle to co-operate with the Republican Governor of New Jersey, Chris Christie, who nurtures his own presidential ambitions.
But despite public backing for bipartisanship, the House of Representatives and the Senate did not change hands, leaving Congress as partisan as before with the Republicans dominating the House and the Democrats retaining the Senate.
But there were still some notable results in congressional races, with Harvard professor Elizabeth Warren scoring an upset victory over Republican senator Scott Brown in Massachusetts. And in Missouri, incumbent Democratic senator Claire McCaskill defeated Todd Akin, a Republican hopeful who embarrassed Mr Romney in August by saying women's bodies shut down to prevent pregnancy after "legitimate rape".
Mr Obama's most immediate challenge will be to try to broker a solution with the recalcitrant Republicans as he looks to his legacy in the next four years.
MORE US ELECTION COVERAGE:
John Armstrong: US continuity will please the Beehive
John Key welcomes Obama's re-election
Republicans seek scapegoats
Map of results
