Washington - Voting started at 6 am (1100 GMT) in several states Tuesday in historic US elections to elect the country's 44th president.

Officials braced for an unprecedented turnout and massive lines at polling places as voters delivered their verdict on Democrat Barack Obama, 47, and his Republican rival John McCain, 72, after the longest and most expensive campaign in US history.

According to tradition, voting actually began at the stroke of midnight Tuesday in a handful of remote towns in the north-eastern state of New Hampshire. The residents of Dixville Notch have been meeting in the town's ballroom at midnight each election day since 1960.

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

Next came Vermont, where one town opened polling stations at 5 am (1000 GMT). Other polls around New Hampshire and Vermont opened at 6 am (1100 GMT) Tuesday, as did polls in Connecticut, Kentucky, New Jersey, New York, Virginia, Maine and Vermont. The last polls will close in Alaska at 0500 GMT Wednesday.

In Norfolk, Virginia, hundreds of people braved pouring rain as they waited patiently from 4:30 am for the polling station at an elementary school to open.

Obama, 47, who would be the first African American president in US history if elected, 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 in the state-by-state, winner-takes-all US system, presidential campaigns focus on key battleground states, and McCain still hopes to pull off an upset victory by winning in states like Florida, Ohio and Pennsylvania.

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.

It is expected that 130 million to 140 million will vote on Tuesday - about 121 million voted in the 2004 elections. According to last available figures from the US Census Bureau from 2004, there are an estimated 215.7 million people of voting age.

This election is widely considered the most important in a generation. Until late Monday, McCain and Obama made last-minute pitches to undecided voters in a race across the country, 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.

It was a bittersweet end to the 21-month campaign for Obama: His grandmother Madelyn Dunham, 86, passed away after a battle with cancer, the Illinois senator revealed Monday.

Despite the polls, 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.

"I'm confident, because I've seen the momentum my friends. All we've got to do is get out the vote," McCain said.

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

Obama will hold his election night rally Tuesday at Grant Park in Chicago - about 1 million people are expected to attend. McCain was to hold his election night event in Phoenix, Arizona.

Although election results are scheduled 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. (dpa)