US polling stations opened on Tuesday, with Democratic incumbent Barack Obama and Republican challenger Mitt Romney locked in a tight presidential contest after a burst of last-minute campaigning.
Polls opened at 6:00 am (1100 GMT) in battleground states New Hampshire and Virginia - either of which could decide the election - as well as New York, New Jersey, Connecticut, Indiana, Kentucky, Maine and Vermont.
After a nearly two-year-long campaign season - the most expensive and one of the most negative on record - Americans will decide whether to re-elect Obama despite the sluggish economy or opt for the change promised by Romney.
Obama is seen as the slight favorite, with a razor-thin advantage in the latest national polls and -- more importantly -- a narrow lead in most of the swing states expected to decide the election.
In US elections the president is not chosen by the popular vote, but indirectly through the electoral college, in which states vote based on population, with a candidate needing 270 out of 538 electoral votes to win.
Tuesday's first votes were cast just after midnight in the tiny hamlet of Dixville Notch, New Hampshire, which boasts the first ballots in US elections.
For the first time ever the vote was tied - with Obama and Romney each receiving five votes.
Early voting has been under way in some states for several weeks.