Donald Trump has claimed victory in the US presidential race, pulling off one of the biggest upsets in US political history.

Mr Trump claimed victory after it became clear he had secured the 10 electoral votes in Wisconsin, tipping him past the mark of 270 needed to head to the White house.

Democrats candidate Hillary Clinton, who began the day as unbackable favourite, was not prepared to make a concession speech, however, with her campaign chairman John Pedesta emerging shortly after 2am (6pm AEDT) to say the election was too close to call.

“We can wait a little bit longer can’t we?” Mr Podesta said on stage at the Clinton HQ in New York.

In a move reminiscent of John Kerry’s 2004 loss, Ms Clinton was of the opinion that several states were too close to call.

“‘So we won’t be saying anything more tonight,” Mr Podesta said. He finished by saying ”let’s get these votes counted” and ”let’s bring this home!”.

It was later reported, however, that she privately phone Mr Trump to concede.

Wisconsin gave Mr Trump the electoral votes he needed to claim victory, having earlier won the critical states of Ohio, Florida, North Carolina and Pennsylvania.

Even before Wisconsin his campaign manager Kellyanne Conway had described the mood inside Trump Tower as “buoyant”, with expectations high after the Republican nominee roped in a string of important swing states.

Thousands of his supporters gathered in a midtown Manhattan hotel ballroom watching the voting unfold on Fox.

While the final result remained in the balance well after midnight, Mr Trump led 276-218, a result many pollsters and pundits did not think possible.

Supporters of Ms Clinton began to voice their concerns about an upset outcome late in the evening.

A Donald Trump supporter celebrates the result in New York. Photo: Getty

CNN reported that Ms Clinton’s supporters at her New York City HQ were “shocked”, “stone faced” and “crying”, with several leaving the venue before an offical result was known.

At 6.30pm AEST, Donald Trump led Hillary Clinton by 58 electoral college votes, according to media reports, with the Republicans also expected to retain control of the US House of Representatives.

Ms Clinton had taken much-needed votes in California, Oregon, Nevada, Maine, New Mexico and Colorado.

There are 538 Electoral College votes allotted to the 50 states and the District of Columbia. It takes 270 votes to win.

With voting completed in more than two-thirds of the 50 US states, the race was too close to call in New Hampshire, Minnesota and Michigan; any of which would assure Mr Trump of the presidency.

The New York Times election forecaster, which had given Ms Clinton a 90 per cent probability of winning in the days leading up to the vote, had flipped to a greater-than-95 per cent chance of Trump entering the White House, while the businessman was listed as the favourite to win on gambling sites such as Bet Fair.

Ms Clinton took the small states of New Jersey and Massachusetts, plus Virginia, Nevada and the expected California and New York, but lost the key state of Ohio – which has predicted the winner of every US election since 1964.

The CNN Exit Poll for Florida, posted just after midday AEST, had fared well for Hillary Clinton, but the key state was called for Mr Trump’s camp at 3pm AEST. 

Gender gap likely to help Clinton in Pennsylvania

The 20 college delegates on offer in Pennsylvania helped smooth Mr Trump’s path to the Presidency.

Exit polls for Pennsylvania had been more encouraging for Ms Clinton, but Mr Trump polled almost 100,000 more votes.

Like Florida there are more women registered to vote than men and 58 per cent of them supported Ms Clinton.

Mr Trump had 54 per cent of the male vote.

Polls began closing at 10am AEST in the US, starting in the east with Dixville Notch in New Hampshire and sweeping across a number of tightly-contested states including North Carolina, Florida and Ohio.

The Hispanic population, known as “the sleeping giant”, proved crucial . Photo: Getty

Florida was named a must-win for Mr Trump by several polls and media commentators who believed the race would be out of reach for the Republican without those key 29 electoral college votes.

Mr Trump began the day continuing to keep the world in suspense on the important question of whether he would accept a Clinton win.

“I will totally accept the results – if I win,” declared the brusque businessman in the lead-up to the vote, while on election day he told a New York radio station everything would need to be “on the up and up” for him to accept a loss.

Ms Clinton had reason to be worried as swing states edged towards Mr Trump. Photo: AAP

As Americans cast their votes, The New York Times reported that women, especially those holding college degrees, had the power to wrangle the election for Ms Clinton, should they continue to show increased turnout compared to the 2012 election.

The same was said for the Hispanic population, referred to by media as “the sleeping giant” that has been woken. Surprisingly, however, about one in three Hispanic males voted for Mr Trump.

Unlike Australia’s preference system, the US operates on a first-past-the-post basis at the federal level – meaning the candidate with the most votes in each state automatically gains all of that states allocated electoral college votes (except in Maine and Nebraska).

From there, the candidate with the absolute majority of college votes is named president.