Washington - Election day finally arrived Tuesday for the record throngs of US voters planning to cast their presidential ballots for either Democrat Barack Obama or Republican John McCain.

Obama held a significant lead in opinion polls as state officials braced for an unprecedented turnout and massive lines at polling places in an election widely considered the most important in a generation.

As per tradition, at the stroke of midnight, voting began in a handful of towns in the north-eastern state of New Hampshire. The residents of Dixville Notch have been meeting in the town's ballroom at midnight since 1960 - the longest running tradition.

Obama won the town's poll by 15 votes to six for McCain in a departure from 40 years of Republican loyalty.

Vermont comes next - one town opens polling stations at 5 am (1000 GMT). Other polls around New Hampshire and Vermont will open at 6 am Tuesday, as will polls in Connecticut, Kentucky, New Jersey, New York, Virginia, Maine and Vermont. The last polls will close in Alaska at 0500 Wednesday.

McCain, 72, and Obama, 47, made last-minute pitches to undecided voters in a race across the country on Monday, reprising familiar themes that helped get them to this final marker of the 21-month campaign.

"After decades of broken politics in Washington ... We are less than one day away from bringing about change in America," Obama told an estimated crowd of 100,000 at his final campaign event in Manassas, Virginia.

Obama, who would be the first African American president in US history, was the strong favourite heading into Tuesday's vote. An aggregate of major national polls compiled by realclearpolitics. com gave Obama 51.6 per cent to McCain's 44.3 per cent on Monday.

But McCain remained upbeat as he ended a seven-state, 5,940- kilometre tour of the country with a midnight rally in Prescott, Arizona, the state he has represented in Congress for more than 20 years and the home town of former Republican presidential candidate Barry Goldwater.

"I'm confident, because I've seen the momentum my friends. All we've got to do is get out the vote," McCain said, promising to get the country "back on track" after eight years of an unpopular President George W Bush.

In a break from tradition, McCain also planned two final rallies on voting day Tuesday in Colorado and Nevada. Obama was set to meet voters in Indianapolis, Indiana on Tuesday before returning to his home town of Chicago.

Obama's choice of Virginia for his last rally Monday night was testament to the changing electoral map in this election. The state has not voted for a Democrat since 1964 but is leaning toward Obama, and supporters were mindful of their important place in history.

"I'm so glad that everyone brought their kids out. This is a special moment," said Antonio Goodman, 31, who travelled to the rally from the US capital Washington.

But it was a bittersweet end to the campaign for Obama: His grandmother Madelyn Dunham, 86, passed away early Monday morning after a battle with cancer. Obama, who was part raised by his grandmother, called Dunham the "cornerstone of our family" paid a tearful tribute to her in North Carolina.

Tens of millions of people have already taken to the polls in recent weeks for early or absentee voting allowed in 31 states, including key battlegrounds Florida, North Carolina, Colorado and Nevada.

Obama was to hold his election night rally Tuesday at Grant Park in Chicago before as many as 1 million people, and unseasonably warm temperatures were expected for the outdoor event.

McCain was to hold his election night event in Phoenix, Arizona, the state he has represented in Congress since 1982.

But with massive voter turnout as high as 90 per cent possible in some precincts, and new equipment in many states, it could be a long night before the winner is declared.

Although election results are expected to start coming in about 0100 GMT Wednesday, western states could also play a big role this year, with results expected to filter in well after 0300 GMT.

Election officials in Chicago said they expected about 80 per cent of eligible voters to come out Tuesday, which would be higher than any time since 1972. (dpa)