Hillary Clinton and Donald Trump have cast their ballots in the US election, with the latter saying it will be a "tremendous waste of time" if he misses out on the White House.
Millions of Americans are going to the polls to elect their 45th president at the end of an acrimonious and wildly unpredictable campaign that has divided the nation.
America will either have its first female commander-in-chief in Mrs Clinton, or choose billionaire tycoon Mr Trump, a flamboyant former reality TV star who has rocked US politics.
Mrs Clinton greeted supporters waiting outside her polling station as she and husband Bill cast their ballot in their home town of Chappaqua, New York.
She said it was "the most humbling feeling" to vote "because so many people are counting on the outcome of this election."
Mr Trump, who arrived at a New York City polling station earlier to cast his vote, expressed confidence he would "win lots of states".
He told Fox News: "If I don't win, I will consider it a tremendous waste of time, energy and money."
Heavily armed police guarded Mr Trump as he voted at Public School 59 in Manhattan, a short distance from Trump Tower.
Hundreds of people watched on outside as the Republican was greeted by a mixture of boos and cries of "Donald, Donald".
Mr Trump - who was accompanied by his wife Melania, daughter Ivanka and son-in-law Jared - said it was a "great honour, a tremendous honour" to be casting his ballot.
He said his final message to voters was: "Make America great again. That's all it is. That's what it's all about."
With most polls showing Mrs Clinton in the lead, US election analyst Todd Donovan told Sky News: "Objectively it's Clinton's election to lose."
According to a poll released before balloting opened, Mrs Clinton has about a 90% chance of defeating Mr Trump.
:: America 2016: Stakes could not be higher
The Reuters/Ipsos States of the Nation survey found Ms Clinton was leading Mr Trump 45% to 42% in the popular vote, and was on track to win 303 votes in the Electoral College to her rival's 235, clearing the 270 needed for victory
he RealClear Politics poll of polls also gives the Democratic nominee a lead of more than 3% over her Republican rival.
The election will be held under tight security, after authorities received intelligence of a possible pre-election al Qaeda attack.
Some 5,000 police officers will be deployed in midtown Manhattan in New York City.
:: Financial markets anxiously await result
Both candidates held a last day of frantic campaigning to energise supporters and win over any undecided voters.
On Monday, flanked by ex-presidents, rock stars and family, the candidates blitzed through a handful of battleground states where the race will be largely decided.
Comments