President Barack Obama has won re-election to a second term in the White House and made a victory speech in which he said the best is yet to come for the United States.
Obama and his family came on stage at the Democrat campaign HQ in Chicago to huge cheers with the song Signed, Sealed, Delivered by Stevie Wonder.
The crowd chanted "four more years".
Obama said: "We are an American family and we rise or fall together as one people and one nation."
He added: "We know in our hearts that for the United States of America the best is yet to come."
He paid credit to Republican challenger Mitt Romney, who earlier conceded defeat, and he praised his wife and daughters.
Electoral progress score (with only Florida to come)
To win : 270
Follow live updates .
He vowed to listen to both sides of the political divide in the weeks ahead and said he would return to the White House more determined than ever to confront America's challenges.
"Whether I earned your vote or not, I have listened to you, I have learned from you and you have made me a better president," Obama said.
The nationwide popular vote remained extremely close with Obama
taking about 50% to 49% for Romney, after a campaign in which the
candidates and their party allies spent a combined US$2 billion
Obama defeated Romney in a series of key swing states despite a weak economic recovery and persistent high unemployment as US voters decided between two starkly different visions for the country.
Obama's victory in the hotly contested swing state of Ohio - as projected by TV networks - put him over the top in the fight for the 270 electoral votes needed to clinch the White House and ended Romney's hopes of pulling off a string of swing-state upsets.
ONE News Political Editor Corin Dann said the mood at the Obama HQ in Chicago was rapturous, while US Correspondent Jack Tame said the picture in Boston was very different with supporters leaving in droves.
Some Republicans had questioned whether Obama had in fact won the key state of Ohio despite the decisions by election experts at all the major TV networks to declare it for the president. But Romney later phoned Obama to concede defeat.
"I believe in America, I believe in the people of America and I ran for office because I am concerned for America," he said in a concession speech in Boston.
He said he wished he had fulfilled the hopes of his supporters, but now prayed for Obama. "This is a time of great challenges for America and I pray that the president will be successful in guiding our nation."
Networks projected Obama as the winner in a number of other swing states, including Colorado, making the Ohio result less decisive.
Obama scored narrow wins in Ohio, Wisconsin, Iowa, Pennsylvania and New Hampshire - all states that Romney had contested - while the only swing state captured by Romney was North Carolina, according to network projections.
At least 120 million American voters had been expected to cast votes in the race between the Democratic incumbent and Romney after a campaign focused on how to repair the ailing US economy.
Obama enters his second four-year term faced with a difficult task of tackling US$1 trillion annual deficits, reducing a US$16 trillion national debt, overhauling expensive social programs and dealing with a gridlocked U.S. Congress that looked likely to maintain the same partisan makeup.
Obama's projected victory would set the country's course for the next four years on spending, taxes, healthcare, the role of government and foreign policy challenges such as the rise of China and Iran's nuclear ambitions.
Each man offered different policies to cure what ails America's weak economy, with Obama pledging to raise taxes on the wealthy and Romney offering across-the-board tax cuts as a way to ignite strong economic growth.
Inside Obama's Chicago campaign headquarters, staffers erupted into cheers and high fives as state after state was called for the president.
Obama watched the returns on television at his Chicago home. Senior campaign strategist David Axelrod said via email that he was feeling "great".
Romney made last-minute visits to Ohio and Pennsylvania today to try to drive up turnout in those states, while Vice President Joe Biden was dispatched to Ohio. Obama remained in his hometown of Chicago.