After a bitterly fought campaign, voting is underway across the United States to decide on the country's 45th President.
Analysts expect voter turnout to be a key factor in whether Donald Trump or Hillary Clinton wins the presidency.
A record 45 million people have cast their ballots ahead of election day.
All 50 states plus Washington DC will vote across 6 different time zones, with the first polls in most of Florida, South Carolina and Virginia closing at 11am (AEDT), California at 3pm (AEDT) and Alaska at 4pm.
A judge has rejected a Trump campaign lawsuit to preserve votes at four Las Vegas polling places for allegedly breaching laws by allowing people to vote after the lines had closed.
Earlier, Republican candidate Donald Trump voted near his home in Manhattan in New York city with his wife Melania, at one point looking over into her voting booth to see how she had voted.
“Not nervous. I'm very excited. It's a great opportunity. It's there's tremendous enthusiasm from everybody you see it all over the country. All over the world. It's been a great feeling. (Reporter:)"Who did you vote for?" A tough decision."
Hillary Clinton voted in upstate New York.
"It is the most humbling feeling Dan because you know how much responsibility goes with this and so many people are counting on the outcome of this election, what it means for our country and I will do the very best if I can if I'm fortunate enough to win today. There have been long queues in the battleground state of Virginia where a record turnout is predicted."
Voters in Virginia meanwhile have been explaining the reason for their choice of president.
"Trump's whole spiel* over the past couple of months has really changed my mind. I wasn't going to vote but I decided to come out there and vote Trump."
"I think it was a very dirty campaign. The media was very involved. Apparently the candidates were not well liked so I'll be glad when it's over."
"I voted for Hillary - I'm a registered Republican and I'm very very disappointed in the development of our party and looking at the two candidates there really was only one option - she's qualified."
"I voted for Donald Trump because, to be honest you know I'm not really educated in the candidates so I didn't really know, I'm clueless."
Former U-S president George W. Bush has cast his vote in this year's election but a spokesperson says he did not vote for either Hillary Clinton or Donald Trump.
Many voters say they chose to vote for a third-party presidential candidate despite reservations that their vote might end up helping a candidate they don't want to see in office.
Brian Wilcher, from Portland in Oregon, explains his reason for voting for the Green Party's Jill Stein.
"Mainly, I chose to vote for Jill Stein because I want to see the two party system end in this country and I know that if a third party gets at least 5 percent of the vote they will get federal funding for future elections so that's why I chose to do that, and I'll be perfectly honest, I have the luxury because I know that this state's going to go blue, no matter what. If I was in a swing state I probably would've sucked it up and voted for Hillary."
While the world has focussed on the presidential election another big campaign has been underway for seats in the US Congress.
All 435 members of the House of Representatives are up for election, so are 34 of the 100 Senators.
The outcome will have an impact on how much the new President will be able to do.
Democrats are slightly favoured to take control of the US Senate from the Republicans - but analysts say it will be tight.
Voters will also choose Governors for 12 states.
In Australia, Deputy opposition leader Tanya Plibersek told ABC radio she's hoping for a win for Hillary Clinton.
Senior Cabinet minister Christopher Pyne, too, has told Channel 10 a win for Ms Clinton would be the best outcome for Australia.
"Hillary will win and win easily and I think that will be the best outcome for Australia because she does support free strayed, she does support the United States -- free trade, she does support the United States being engaged in the Asian region, critical to us. She understands America's place in the world."
Independent senator Nick Xenophon says he's seeking a mates' rates deal on building a bunker if Mr Trump becomes president, referring to the president's authority to issue nuclear weapons.
