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

America's first black president secured the 270 votes in the electoral college needed to win the race.

In his victory speech before supporters in Chicago, Obama said he would talk to Romney about "where we can work together to move this country forward".

Obama prevailed despite lingering dissatisfaction with the economy and a hard-fought challenge by Romney.

His Democrats also retained their majority in the Senate, which they have held since 2007, while Republicans kept control of the House.

US Presidential Election 2012 Infographics

The BBC's Mark Mardell in Chicago says the president is challenging his opponents, asking them to work with him.

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

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

Obama greeted jubilant supporters and congratulated 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.

Obama said he was returning to the White House "more determined, and more inspired than ever about the work there is to do, and the future that lies ahead".

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, Romney congratulated the president and said he and Ryan had "left everything on the field" and had given their all in the campaign.

Referring to the struggling economy, Romney said that now was not the time for "partisan bickering and political posturing", and that Republicans and Democrats must "put people before politics".

"I so wish that I had been able to fulfil your hopes to lead the country in a different direction but the nation chose another leader and so I join with you to earnestly pray for him and for this great nation," he said.

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.

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

Romney won North Carolina and Indiana, both of which 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.

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