President Barack Obama has been re-elected to a second term, defeating Republican challenger Mitt Romney.

With results in from most states, America's first black president has secured the 270 votes in the electoral college needed to win the race.

Mr Obama prevailed despite lingering dissatisfaction with the economy and a well-funded challenge by Mr Romney.

He won by a comfortable margin, even while the swing state of Florida remains too close to call.

With Florida's 29 electoral votes still undecided, Mr Obama won 303 electoral votes to Mr Romney's 206.

In Chicago, Mr Obama greeted cheering supporters and congratulated Mr Romney and Republican Vice-presidential candidate Paul Ryan on their 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," he said.

He pledged to work with Republican leaders in Congress to reduce the government's budget deficit, fix the tax code and reform the immigration system.

"We are an American family and we rise and fall together as one nation," he said.

In Boston, where his campaign was based, Mr Romney congratulated the president and said he and Mr Ryan had "left everything on the field" and had given their all in the campaign.

"This election is over, but our principles endure," he said. "I so wish that I had been able to fulfil your hopes to lead the country in a different direction."

Under the US constitution, each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-takes-all state contests - becomes president.

The popular vote, which is symbolically and politically important but not decisive in the race, remains too close to call.

On Tuesday, the president held the White House by assembling solid Democratic states and a number of important swing states such as Colorado, Iowa, Pennsylvania, Michigan, Minnesota, Virginia and Wisconsin. His narrow victory in Ohio, a critical Mid-Western swing state, sealed the victory.

Billions spent

Mr Romney won North Carolina and Indiana, both of which Mr Obama won in 2008, as well as the solid Republican states.

But he was unable to win in Ohio or other states needed to breach the 270 threshold.

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.

Republicans are projected to keep control of the House, while Democrats are tipped to remain in control in the Senate.

Mr Obama's victory came despite lingering high employment - 7.9% on election day - and tepid economic growth.

But voters gave him credit for his 2009 rescue of the US car industry, among other policy accomplishments, and rewarded him for ordering the commando mission that killed Osama Bin Laden in Pakistan last year.

He and Mr Romney, as well as their respective allies, have spent more than $2bn (£1.25bn) - largely on adverts in swing states.