Polls have closed across half the US as Hillary Clinton and Donald Trump await the verdict of voters after a bitterly fought White House election campaign.
Republican nominee Mr Trump has racked up wins in southern states and Democrat Mrs Clinton has swept northeastern states, ABC News projects.
But the races are too close to call in the vital swing states of Virginia, North Carolina, Florida and Ohio.
A BBC opinion poll had put Mrs Clinton four points ahead nationwide.
As expected, Mr Trump has been victorious in the Republican strongholds of Alabama, Kentucky, South Carolina, Indiana, West Virginia, Mississippi, Tennessee and Oklahoma, ABC News projects.
The network tips Mrs Clinton to have won the Democratic heartlands of New Jersey, Massachusetts, Maryland, Vermont, Delaware, Illinois, Rhode Island and District of Columbia.
A candidate must secure 270 of the 538 electoral college votes to declare victory.
Results are expected some time after 23:00 EST (04:00 GMT on Wednesday), once voting ends on the West Coast.
In other developments:
The North Carolina Board of Elections agreed to extend voting in eight precincts in Durham County amid long queues
Failed Republican presidential candidate Marco Rubio won his bid for re-election in Florida’s Senate race
The Mexican peso dipped after Mr Trump took a razor’s edge lead in the key state of Florida
A shootout involving a female gunman left one dead and three injured near a polling station in the southern California city of Azusa
A record number of Americans – more than 45 million – voted early by post or at polling stations.
Mr Trump, a Manhattan real estate tycoon, and Mrs Clinton, who would be the first US female president, cast their votes in New York City.
Mr Trump was booed as he arrived to vote at a school in Manhattan, alongside his wife, Melania, and daughter, Ivanka.
The two presidential hopefuls will spend election night in New York City, staging events barely more than a mile apart from each other.
Mrs Clinton will address supporters at the Javits Centre in Manhattan, while Mr Trump holds an event at the Hilton Midtown hotel.
More than 5,000 police officers have been deployed across America’s biggest city to keep order on election night.
ABC News has also released initial exit poll data showing 61% of voters viewed Mr Trump unfavourably while 54% said the same of Mrs Clinton.
Earlier on Tuesday, Mr Trump again declined to say whether he will accept the results.
“We’re going to see how things play out,” he told Fox News, while alleging there had been voting irregularities. “I want to see everything honest.”
Some polling locations have reported equipment failures and long lines, but they appeared to be routine problems.
Mr Trump, who stoked conspiracies over the legitimacy of the democratic process during the campaign by claiming the vote would be “rigged”, also filed an emergency lawsuit in Nevada.
The Republican’s camp sued the Clark County Registrar of Voters, alleging the state had kept early voting stations open late.
But a judge rejected the request.
Voting kicked off just after midnight in the small New Hampshire village of Dixville Notch, where a mere seven votes were cast.
Mrs Clinton received four votes while two went for Mr Trump and one for libertarian candidate Gary Johnson.
All 50 states and Washington DC are voting across six different time zones.
Americans also have a chance to determine which party will control Congress.
All 435 seats in the House of Representatives are up for grabs, though it is forecast to remain in Republican hands.
But a third of seats in the Senate, where Republicans also hold sway, are in play, too, and Democrats hope to snatch control of that chamber.
Mr Trump and Mrs Clinton are seeking to succeed Democratic President Barack Obama.
After two four-year terms in the White House, he is barred by the US constitution from running for re-election.
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