The early results on Tuesday were as anticipated, with the contest expected to come down to seven swing states: Georgia, North Carolina, Pennsylvania, Arizona, Michigan, Nevada and Wisconsin.
Opinion polls show the rivals neck-and-neck in all seven going into election day.
As of 8pm ET ( 12pm on Wednesday AEDT), polls had closed in 25 states.
Trump had 90 electoral votes after winning Kentucky, Indiana, West Virginia, Alabama, Florida, Oklahoma, Missouri and Tennessee; Harris had gained 27 electoral votes from Vermont, Maryland, Massachusetts and Washington, DC.
An Edison Research poll shows 73 per cent of voters believe democracy is in jeopardy. (AP PHOTO)
A candidate needs 270 votes in the state-by-state electoral college to claim the presidency.
Nearly three-quarters of voters say American democracy is under threat, according to preliminary national exit polls from Edison, reflecting the nation's deep anxiety after a contentious campaign.
Democracy and the economy ranked by far as the most important issues for voters, followed by abortion and immigration.
The poll shows 73 per cent of voters believe democracy is in jeopardy against 25 per cent who say it's secure.
The data underscores the polarisation in a nation where divisions have only grown starker during a fiercely competitive race.
Trump employed increasingly apocalyptic rhetoric while stoking unfounded fears that the election system cannot be trusted.
There is absolutely no truth to this allegation. It is yet another example of disinformation. Voting in Philadelphia has been safe and secure. — Seth Bluestein (@SethBluestein) pic.twitter.com/wMiPnAgO17November 5, 2024
Harris warned a second Trump term would threaten the underpinnings of American democracy.
Hours before polls closed, Trump claimed on his Truth Social site without evidence that there was "a lot of talk about massive CHEATING" in Philadelphia, echoing his false claims in 2020 that fraud had occurred in large, Democratic-dominated cities.
He later asserted there was fraud in Detroit.
"I don't respond to nonsense," Detroit City Clerk Janice Winfrey told Reuters.
A Philadelphia city commissioner, Seth Bluestein, replied on X: "There is absolutely no truth to this allegation. It is yet another example of disinformation. Voting in Philadelphia has been safe and secure."
Trump, whose supporters attacked the US Capitol on January 6, 2021, after he claimed the 2020 election was rigged, voted earlier near his home in Palm Beach, Florida.
Donald Trump voted near his Florida home, saying he would respect the result of a "fair" election. (AP PHOTO)
"If I lose an election, if it's a fair election, I'm gonna be the first one to acknowledge it," Trump told reporters.
His campaign has suggested he might declare victory on election night even while millions of ballots have yet to be counted, as he did four years ago.
Millions of Americans waited in orderly lines to cast ballots, with only sporadic disruptions reported across a handful of states, including several non-credible bomb threats the FBI said appeared to originate from Russian email domains.
Trump plans to watch the results at his Mar-a-Lago club before speaking to supporters at a nearby convention centre.
Harris was due to address students at Howard University, a historically Black college in Washington where Harris was an undergraduate.
"To go back tonight to Howard University, my beloved alma mater, and be able to hopefully recognise this day for what it is, is really full circle for me," Harris said in a radio interview.
Kamala Harris spent election day doing radio interviews and imploring people to vote. (AP PHOTO)
A dizzying race churned by unprecedented events - two assassination attempts against Trump, President Joe Biden's surprise withdrawal and Harris' rapid rise - remains too close to call after billions of dollars in spending and months of frenetic campaigning.
No matter who wins, history will be made.
Harris, 60, the first female vice-president, would become the first woman, Black woman and South Asian American to win the presidency.
Trump, 78, the only president to be impeached twice and the first former president to be criminally convicted, would also become the first president to win non-consecutive terms in more than a century.
Control of both chambers of Congress is also up for grabs.
Democrats have only a narrow path to defend their Senate majority after Republican Jim Justice flipped a West Virginia seat on Tuesday, while the House of Representatives looks like a toss-up.
