Barack Obama has won a second term as president of the United States, beating Republican challenger Mitt Romney after a long and bitter campaign.
With results in from most states, Mr Obama secured the 270 votes in the electoral college needed to win the race.
Mr Obama defeated Mr Romney in a series of key swing states on Tuesday, despite a weak economic recovery and persistent high unemployment, as an estimated 120 million people voted.
The president's narrow wins in the crucial state of Ohio, as well as Wisconsin, Iowa, Pennsylvania and New Hampshire - all states his rival had contested - effectively ended Mitt Romney's hopes of capturing enough electoral college votes needed to move into the White House, the BBC reports.
Mr Obama's margin of victory is not yet certain. All the votes have not been counted in Florida yet and a result is due on Wednesday, but Mr Obama has won 303 electoral votes to Mr Romney's 206.
The Democrats also retained control of the Senate, which they have held since 2007, while Republicans kept control of the House of Representatives.
Barack Obama made history by becoming America's first black president after a euphoric victory in 2008 and awaited the results in his hometown of Chicago where he delivered his victory speech.
Late on Tuesday he thanked supporters at his party's headquarters, walking out on a stage with his wife Michelle and daughters Sasha and Malia to a cheering crowd. He congratulated Mitt Romney and vice-presidential candidate Paul Ryan on a hard-fought campaign.
"We have picked ourselves up, we have fought our way back and we know in our hearts that for the United States of America the best is yet to come."
Mr Obama reached out to everyone, saying most shared certain hopes for America's future.
"We want our kids to grow up in a country where they have access to the best schools and the best teachers. A country that lives up to its legacy as the global leader in technology, discovery and innovation with all the good jobs and new businesses that follow.
"We want our children to live in America that isn't burdened by debt, that isn't weakened by inequality, that isn't threatened by the destructive power of a warming planet."
The 51-year-old enters his second four-year term faced with a difficult task of tackling $US1 trillion annual deficits, reducing a $US16 trillion national debt, overhauling expensive social programmes and dealing with a gridlocked US Congress that is likely to maintain the same partisan make-up.
Mr Obama pledged to work with Republican leaders in Congress to reduce the government's budget deficit, fix the tax code, reform the immigration system and reduce the nation's dependence on foreign oil. However, he said it would not be easy and compromise is needed.
Romney campaigns till the end
Mitt Romney a multi-millionaire former investment manager and Massachusetts governor, continued campaigning to the end, visiting Ohio and Pennsylvania on Tuesday.
As well as the solid Republican states, he won North Carolina and Indiana.
The 65-year-old told supporters at an event in Boston later that he had called Barack Obama. In a short but gracious concession speech, he congratulated Mr Obama and all involved in the campaign.
"I wish all of them well, but particularly the President, the First Lady and their daughters. This is a time of great challenges for America and I pray that the President will be successful in guiding our nation."
Mr Romney apologised to his supporters, thanked his wife Ann and said he and vice-presidential candidate Paul Ryan had given their all.
"This election is over, but our principles endure. I so wish that I had been able to fulfil your hopes to lead the country in a different direction."
There were long lines at polling places in many states on Tuesday, despite more than 30 million people casting their ballots before election day.
The first votes were cast and quickly counted in the village of Dixville Notch in New Hampshire, resulting in a tie with five votes each for Mr Obama and Mr Romney.
Turnout was heavy in areas of New Jersey and New York that were damaged by super-storm Sandy a week ago.
Lengthy waits were reported in Florida, Virginia and Ohio, as well as New Jersey and New York. In the Miami area, wait times were as long as three hours.
How election is decided
The US election is decided by the electoral college. Each state has a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes becomes president.
Also on Tuesday's ballot were 11 state governorships, a third of the seats in the 100-member US Senate and all 435 seats in the House of Representatives, the BBC reports.
The 2012 campaign began nearly two years ago and is estimated to have cost more than $US2 billion between both parties.
More than 130 million people voted in the 2008 presidential election.