Obama re-elected

President Barack Obama has been re-elected for a second term having crossed the 270 electoral college votes despite uncertainty over the economy, which weighed uppermost in the minds of the electorate.

Republican challenger Mitt Romney called President Obama to concede the presidency in a bitterly-fought election after the president won the electoral college seats in crucial swing state of Ohio Virginia, Nevada, Iowa and Colorado, moving past the 270 votes needed for victory.

Paradoxically, while Obama won the electoral college majority, Romney had polled more than a million popular votes over the president.

Under the US constitution, each state has a number of electoral votes roughly proportional to its population and the candidate who gets a minimum 270 of the 538 electoral votes becomes president.

The popular vote, which is too close to call, has symbolic importance and is not decisive in race.

Obama, in a tweet claimed victory in his re-election battle with posts on his Twitter and Facebook pages. "This happened because of you. Thank you. Four more years," signing off with his initials "bo" to mark it as a personal message.