Unusually large numbers of voters turned out to choose between US President George W Bush and Democratic Senator John Kerry after a deadlocked White House campaign.
With the electorate polarised over the country's direction three years after the September 11th, 2001, attacks inspired a short-lived unity, as many as 125 million Americans were expected to vote, making it the biggest turnout in more than 40 years.
Some polling stations reported long queues of hundreds of citizens outside, hours after polling had officially finished. Those already queuing were allowed to vote.
Long lines formed at polling stations and voters were unusually motivated amid lingering bitterness over the 2000 election, when Mr Bush lost the popular vote to Democrat Mr Al Gore but narrowly won the Electoral College after the US Supreme Court stopped the vote recount in Florida.
As in 2000, the focus today will be on Florida along with the new showdown state of Ohio, both won by Mr Bush four years ago and the biggest toss-up states in the quest for the 270 Electoral College votes needed for victory.
Officials in Florida, where more than 10 million people are eligible to vote, reported long lines but no early problems at polling stations.
There were scattered reports of voting difficulties and irregularities elsewhere, including slashed tyres on cars that were to take voters to the polls in Milwaukee, Wisconsin, and allegations of ballot stuffing in a poor black Philadelphia neighbourhood. The Philadelphia allegations were quickly denied by the District Attorney.
Republicans and Democrats said the election outcome may not be known quickly, and both sides fielded armies of lawyers to challenge close results and girded for the possibility of another long legal fight like the five-week battle in 2000.
Mr Bush said he was confident after voting at the firehouse near his Crawford, Texas, ranch and said the election boiled down to whom voters trusted to secure the country.
"I believe I'm going to win," Mr Bush said before flying to the battleground state of Ohio to call voters from a Republican phone bank and then returning to Washington. "My hope, of course, is that this election ends tonight."
"I've given it my all," he said.
Mr Kerry met Democratic activists in La Crosse, Wisconsin, a battleground state that is one of the few to allow same-day registration, and hammered Mr Bush over the Iraq war.
"You have a choice, all Americans have this choice today," the Massachusetts senator said. "George Bush made his choices. . . . He made a choice without a plan to win the peace."
"I am very confident we made the case for change," he later said after voting at the Massachusetts State House in Boston.
Mr Kerry must win either Florida or Ohio to have a realistic shot at victory, while a Bush loss in Florida would leave him in danger unless he took Pennsylvania or some states such as Minnesota, Wisconsin and Iowa - all won by Gore in 2000.
Independent candidate Mr Ralph Nader could play the role of spoiler, although his support in polls this year is much less than in 2000, when some say he helped tip the race to Mr Bush.
In addition to the presidential race, Americans will also decide which party holds power in Congress and will vote on governorships in 11 states, with Bush's Republicans favoured to retain control of the Senate and House of Representatives.
More than 105.4 million Americans voted in 2000, representing 51.21 per cent of the voting-age population, but as many as 20 million more could vote in the current election.
According to Federal Election Commission figures, the highest turnout in recent decades was in 1960, when about 63 per cent of the voting-age population cast ballots.