Washington/Chicago  - Hundreds of US voters waited patiently in serpentine queues Tuesday to vote in the country's historic election. Many had started lining up before dawn, some braved pouring rain to cast their ballot.

Officials were prepared for an unprecedented turnout 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.

Obama returned to Chicago, the city where his rise to prominence began, to cast his vote at 8:45 am (1345 GMT). His running mate Joe Biden voted moments later in Wilmington, Delaware.

Former president Bill Clinton and his wife Hillary - who lost out on the race to the White House - voted in Chappaqua, New York. "It's really for me a tremendous opportunity and honour to be part of what I hope will be a great couple of years for America," she said.

According to tradition, voting 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 ballot room 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 stood in a relentless downpour as they waited from 4:30 am for polling stations to open.

About 300 voters stood in a line that refused to move outside the New York University dorms in Manhattan - an unusual sight in a city that is constantly moving at a frantic pace. Janitors in a nearby apartment block said they couldn't get off work in the morning, but were looking forward to voting in the afternoon.

Voting was an emotional experience for Rick Garcia in Florida. "I'm voting for my brother (who was killed in Afghanistan). This is what he would have wanted," he told CNN. Garcia said neither he nor his family had voted before this election.

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. If elected, Obama would be the first African American president in US history.

But in the state-by-state, winner-takes-all US system, presidential campaigns focus on key battleground states, and McCain was still hoping to pull off an upset victory by winning in states like Florida, Ohio and Pennsylvania.

Millions already took to the polls in recent weeks for early or absentee voting allowed in 31 states, including key battlegrounds Florida, North Carolina, Colorado and Nevada.

An estimated 140 million people are expected to vote Tuesday - about 121 million voted in the 2004 elections. According to last available figures from the US Census Bureau from 2004, there are 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.

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

Obama's choice of Virginia for his last rally 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.

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.

In a break from tradition, McCain also planned two final rallies on voting day Tuesday in Colorado and Nevada. He was scheduled to vote in Phoenix, Arizona at 11 am
(1600 GMT).

Obama will spend election night in Chicago, where officials expect up to a million people to gather in Grant Park to watch the results and hear from their candidate.

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)