Barack Obama was crowned president of America for another four years early today after seeing off the challenge of Mitt Romney in conclusive style.

At 4.20am UK time, the crucial swing state of Ohio was called for Obama and its 18 electoral college votes pushed him easily through the 270 vote threshhold, and Democrats launched a jubilant party at their Chicago HQ.

By 7am today, Obama had notched up 303 votes against 206 for Romney, with Florida (29 votes) still to declare, but the Democrats ahead there by a narrow margin.

No US president since Franklin D. Roosevelt in the 1930s has been re-elected with a national jobless rate as high as it is now - 7.9% in October.

Obama, making his last run for office at age 51, credits his car industry bailout, stimulus plan and other policies for ending the recession and bring a slow but steady drop in the unemployment rate. Romney, 65, said Obama's policies had kept the economy at a standstill.

Obama thanked supporters on Twitter, saying: "We're all in this together. That's how we campaigned."

He also posted a picture of himself hugging wife Michelle, with the tweet: "Four more years." Within 22 minutes, it had become the most popular tweet of all time.

Obama also sent an email to supporters signed up to his campaign. In it, he said: "Friend, I'm about to go speak to the crowd here in Chicago, but I wanted to thank you first.

"I want you to know that this wasn't fate, and it wasn't an accident. You made this happen. You organised yourselves block by block. You took ownership of this campaign five and ten dollars at a time. And when it wasn't easy, you pressed forward.

"I will spend the rest of my presidency honouring your support, and doing what I can to finish what we started. But I want you to take real pride, as I do, in how we got the chance in the first place.

"Today is the clearest proof yet that, against the odds, ordinary Americans can overcome powerful interests. There's a lot more work to do. But for right now: Thank you. Barack"

Later, Obama, flanked by his smiling wife Michelle and their daughters Malia and Sasha, strode to a lectern at his campaign headquarters in Chicago, Illinois to the upbeat strains of Stevie Wonder's Signed, Sealed, Delivered, I'm Yours.

Looking visibly relieved after many long weeks of hard campaigning, the president looked on as the chant "Four more years, four more years!" resounded across the room.

His victory speech lasted for almost 25 minutes, but one message underlined his every word: "The best is yet to come, but we have more work to do."

In an impassioned speech, he urged voters of all political hues to work together to help solve America's collective problems, and said he wanted to have talks with Romney to discuss how they could work together in the aftermath of the election.

Romney, a former Massachusetts governor, pushed the result to the wire, taking Indiana from his rival. But key battlegrounds, including Ohio and New Hampshire, went to Obama.

Romney called Obama to concede defeat 90 minutes after the result was called, and later took to the stage at party HQ in Boston, where he congratulated the president and apologised to supporters for failing to deliver victory.

Romney, a Mormon, said Obama faced major challenges, adding: "I pray the president will be successful in guiding our nation."

David Cameron also took to Twitter, saying: "Warm congratulations to my friend @BarackObama. Look forward to continuing to work together."