Drama erupted late in the night (US time) with President Donald Trump falsely claiming victory in the presidential election, saying the result would be taken to the US Supreme Court and declaring the vote 'a fraud'.
Trump and Democratic challenger Joe Biden remain locked in a tight presidential race tonight, with the challenger saying earlier "we're on track" to win.
Trump spoke at the White House at 2.21am, saying it was the latest news conference he had ever had, and claimed he had won the election.
"This is a fraud. We are going to the US Supreme Court.
"We were winning everything and all of a sudden it was all called off," Trump said.
"We won states that we weren't expected to win - Florida ... we won it by a lot. The great state of Ohio. Texas.
"We won Texas by 700,000 votes and they don't include it in the tabulation.
"And Georgia... they'll never catch us. They can't catch us
"Arizona ... we have a lot of life in that ... there were a lot of votes out there we could get.
"We don't need Arizona. That's a state that would have been nice. There's a good possibility ... the numbers have come down. We want that to stay in play.
"Most importantly we are winning Pennsylvania by a tremendous number of votes.
"We are up ... think of this, think of this ... up 690,000 votes in Pennsylvania ... that's not close.
"We are winning Wisconsin ... and we don't need to win all of them.
"All of a sudden I said... what happened to the election
"They said ... let's go to court ... they were either going to win or if they weren't going to win they want to go to court.
Biden earlier told his supporters "we're on track" to win the election but "we will have to be patient" until all votes are counted. He said a result might be known tomorrow morning (US time) but "it may take a little longer".
Trump early tonight tweeted "we are up big but they are trying to steal the election. We will never let them do it ..."
He said votes cannot be cast after the polls have closed. AP reports that in multiple states, ballots can in fact be counted if they arrive after Election Day.
Trump later deleted the tweet - seemingly to correct a spelling error in which he wrote "poles" instead of "polls" - and said he would make a statement tonight: "A big win!"
Twitter flagged Trump's tweet about the Democrats "trying to steal the election", saying "some of the content is disputed". And Facebook put a label on the post, saying votes were being counted and no winner was yet projected.
Trump was at long odds with bookmakers to win yesterday, however that changed on Tuesday afternoon (NZ time) and he was listed as the favourite with the Sportsbet and Betfair agencies. By Tuesday evening, the odds had narrowed again and the candidates were almost even.
From coast to coast, races were tight in many states.
Biden picked up the first battleground state of the night, New Hampshire, a small prize that Trump tried to steal from Democrats.
He also won California, the nation's biggest electoral haul, and other predictable victories including Colorado and Virginia, two former battlegrounds that have become Democratic strongholds.
Trump's wins included Kansas, North Dakota and other conservative bastions.
Here are the states that have been called so far by various major news outlets in the US:
For Trump: Arkansas, Florida, Indiana, Kentucky, West Virginia, South Carolina, Alabama, Mississippi, Oklahoma, Nebraska, Wyoming, South Dakota, North Dakota, Kansas, Louisiana, Idaho, Utah, Missouri and Tennessee.
For Biden: Vermont, Virginia, Massachusetts, Maryland, Delaware, Illinois, Connecticut, New York, New Jersey, New Mexico, Rhode Island, New Hampshire, Washington, Oregon, California (which has 55 electoral votes, the biggest of any state), Colorado and the District of Colombia.
Biden is now expected to win Arizona and "that dramatically narrows Trump's pathway to reelection", Fox News said.
The other key states left to be decided are Pennsylvania, Michigan and Wisconsin.
Results in several key battleground states are in flux as election officials processed a historically large number of mail-in votes.
Democrats typically outperform Republicans in mail voting, while the GOP looks to make up ground in Election Day turnout. That means the early margins between the candidates could be influenced by which type of votes — early or Election Day — are being reported by the states.
While there are 50 states in America, when the votes are counted just a handful of them – Florida included – decide who wins the presidential election.
In this election, 15 states fell into the category of a "swing state" or "battleground state" – meaning they could conceivably be won by either Trump or Biden, news.com reports.
Those states were Arizona, Colorado, Florida, Georgia, Iowa, Michigan, Minnesota, New Hampshire, Nevada, North Carolina, Ohio, Pennsylvania, Texas, Virginia and Wisconsin.
Millions of voters put aside worries about Covid-19 — and long lines — to turn out to vote today. They joined 102 million Americans who voted earlier, a record number that represented 73% of the total vote in the 2016 election.
Because of the huge volume of mail-in votes, the outcome may not be known for days or even weeks and could wind up in court.
In downtowns ranging from New York to Denver to Minneapolis, workers boarded up businesses lest the vote - or uncertainty about the winner - lead to unrest of the sort that broke out earlier this year amid protests over racial inequality.
Those who are voting in person on Tuesday (Wednesday NZT) are joining 102 million Americans who voted early, a record total that represents 73 per cent of the total turnout of the 2016 presidential election.
Biden entered election day with multiple paths to victory while Trump, playing catch-up in a number of battleground states, had a narrower but still feasible road to clinch 270 Electoral College votes.
Control of the Senate is at stake, too: Democrats need to net three seats if Biden captures the White House to gain control of all of Washington for the first time in a decade. The House is expected to remain under Democratic control.
Trump began the day on an upbeat note, predicting that he'd do even better than in 2016, but during a midday visit to his campaign headquarters, spoke in a gravelly, subdued tone.
"Winning is easy," Trump told reporters. "Losing is never easy, not for me it's not."
Trump left open the possibility of addressing the nation on Tuesday, even if a winner isn't yet determined. Biden, too, promised a speech.
The Democratic nominee kept his eyes on the critical state of Pennsylvania, taking his final pitch to voters in his hometown of Scranton and the Democratic stronghold of Philadelphia.
In battlegrounds, including Florida, Iowa, Georgia, Michigan and Pennsylvania, some voters showed up to their polling places before dawn to beat the crowds, but still found themselves having to wait in long lines to cast their ballots.
A new anti-scale fence was erected around the White House. And in downtowns ranging from New York to Denver to Minneapolis, workers boarded up businesses lest the vote lead to unrest of the sort that broke out earlier this year amid protests over racial inequality.