The polling stations across parts of the country started to open at 6am (1100 GMT) on Tuesday.
About 156 million Americans are eligible to vote in one of the tightest presidential elections in recent history. Although voter turnout is unpredictable, analysts believe it will be markedly higher than the 106 million in 2000.
Under the US electoral system, however, it is the candidate who wins at least 270 out of 538 Electoral College votes who wins the election - in 2000 Bush won the Electoral College vote despite losing the popular vote by more than half a million votes to then vice-president Al Gore.
Edwards says they feel very
confident, very optimistic
"Young people are voting in very big numbers and that's a very good sign for John Kerry," Democratic vice-presidential candidate John Edwards told NBC television.
"I think we're going to see unprecedented turnout. We feel very confident, very optimistic," he added.
Polling stations opened in nine eastern states: Virginia, New Hampshire, Connecticut, New Jersey, Maine, Vermont, New York, Indiana and Kentucky.
The first polling stations close at 7pm eastern time (0000 GMT on Wednesday) in Georgia, Indiana, Kentucky, South Carolina, Vermont and Virginia. The last polling stations close in Alaska at 0600 GMT on Wednesday.
"We both actually feel very confident about the election, but whatever happens, we'll be great"
wife of President George Bush
Opinion polls show the race in a virtual dead heat. Five surveys gave Bush a statistically insignificant lead of one or two points, while Fox News showed Kerry leading by two points and the American Research Group had a 48-48% tie.
First Lady Laura Bush, asked if she and the president had discussed what they would do with their lives if he did not win re-election, told NBC:
"We both actually feel very confident about the election, but whatever happens, we'll be great."