Voters across the United States are flocking to polling stations to choose a new president.
Republican John McCain is attempting to defy the opinion polls, while Democrat Barack Obama is seeking to become the country's first black president.
Polls close in six eastern states at 0000 GMT, with early indications of their results expected soon afterwards.
Several key states are reporting a heavy turnout. A total of 130 million Americans are expected to vote.
If that figure is confirmed, turnout will be higher than for any election since 1960. About 29 million have voted early.
In Virginia, a Republican stronghold which Mr Obama is hoping to capture, Secretary of State Jean Jensen told reporters: "It's a phenomenal turnout."
Missouri, another battleground state, is reporting an "unprecedented turnout".
Officials in Ohio are expecting about 80% voter participation.
Queues could require some polling stations to remain open late.
There were also reports of glitches with electronic voting machines in states, including Florida, Ohio and Virginia - though not on a big scale.
Mr Obama and his wife voted at a polling station in Chicago, Illinois, with their two daughters.
"When polls close, the journey ends but voting with my daughters, that was a big deal," he said afterwards.
Mr McCain cast his ballot in Phoenix, Arizona.
Unusually, both candidates continued to campaign during polling day - Mr Obama in Indiana and his rival in Colorado and New Mexico.
Mr McCain urged supporters to "get out there and vote", and "drag" their neighbours to polling stations if need be.
Many Americans said they felt they were voting in a historic election, not least because of the possibility of choosing the first African-American president.
Faton Fall, 40, a black voter queuing at a Baptist church in Chicago, said: "It means a lot to me. I'm overwhelmed. I can't say more."
In the first voting of the day, Mr Obama won by 15 votes to six in Dixville Notch, New Hampshire.
The hamlet, which has a 60-year tradition of being first in the nation to vote, opened its polls at midnight, with a 100% turnout.
It was the first time the town had voted for a Democrat since 1968.
There are also elections to renew the entire US House of Representatives and a third of US Senate seats.
Democrats are expected to expand majorities in both chambers.
They need to gain nine Senate seats to reach a 60-seat majority that would give them extra legislative power.
The final Reuters/C-SPAN/Zogby poll of the election published on Tuesday found likely voters favoured Mr Obama by 11 points over Mr McCain, 54-43%.
Other national polls indicate Mr Obama increasing his lead over his rival to as much as 13 points.
But the BBC's James Coomarasamy in Washington says that while Mr Obama has held a consistent lead for several weeks, a number of factors could undermine the pollsters' predictions.
Among them, he says, are the role the Illinois senator's skin colour may play in voters' intentions; whether newly registered voters will actually vote; and the Palin effect - whether Mr McCain's running mate has energised or alienated Republicans.
Under the US Electoral College system, states are allocated votes based on their representation in Congress.
In almost every state, the winner gets all these college votes.
To become president, a candidate needs to win a majority across the country - 270 college votes out of a possible 538.
BBC North America editor Justin Webb says there is much interest in three well-populated swing states - Florida and Ohio, both won narrowly by George Bush in 2004, and Pennsylvania, which went to the Democratic candidate John Kerry.
If Mr Obama can take Florida or Ohio, he is sure to become president, our correspondent says.
If John McCain holds them and takes Pennsylvania, he could just win, our correspondent adds.
The presidential election has been the most expensive in US history - costing $2.4bn, according to the non-partisan Center for Responsive Politics.
On the eve of the poll, Mr Obama said his grandmother, Madelyn Dunham - who largely raised him as a child - had died aged 86 in Hawaii after losing her battle with cancer.
Originating at news.bbc.co.uk