A final result could still take hours or even days to materialize if the margins in Pennsylvania, Georgia, Arizona and the other swing states come down to a few thousand votes at a time.

Millions of Americans lined up throughout Election Day -- and millions more voted early -- in a race with momentous consequences for the United States and the world.

The anxiously awaited outcome will either make Harris the first woman in the world's most powerful job or hand a historic comeback to Trump and his right-wing "America First" agenda.

In a stark reminder of the tension -- and fears of outright violence -- dozens of bomb threats were made against polling stations in the swing states of Georgia and Pennsylvania.

The FBI said the threats appeared to originate in Russia, which is accused by Washington of trying to meddle in the election.

The threats were all hoaxes but succeeded in disrupting proceedings.

Voting was temporarily suspended at five locations in the majority Black, Democratic stronghold of Fulton County in Georgia -- a key bastion for Harris. In Pennsylvania, Governor Josh Shapiro said that "thus far, there is no credible threat to the public."

- 'Big victory' -

Trump -- who has still refused to accept his 2020 election loss, after which his supporters attacked the US Capitol -- added as the first results came in that "we're going to have a big victory tonight."

Harris urged people to vote as she spent the day in Washington doing interviews with radio stations and taking a few calls personally at a phone bank for voters.

"We've got to get it done. Today is voting day, and people need to get out and be active," Harris told Atlanta station WVEE-FM.

Trump had an early lead, partly thanks to projected wins in reliably Republican Florida, Texas and Ohio, giving him 201 electoral votes to Harris's early haul of 90.

She was likely to get a big boost when the country's biggest and reliably Democratic state, California, comes in. However in the end, all will depend on the battlegrounds: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

In a possible preview of coming election challenges, Trump took to social media to say there is "talk about massive cheating" in Philadelphia, the Democratic stronghold of vital Pennsylvania.

City officials rejected the charge.

There were also fears of violence if Trump loses and numerous buildings in central Washington were boarded up on Tuesday.

Polls for weeks have shown a knife-edge race between Harris and Trump, who at 78 would be the oldest ever president at the time of inauguration, the first felon president, and only the second in history to serve non-consecutive terms.

Harris, 60, would also be only the second Black and first person of South Asian descent to be president.

She made a dramatic entrance into the race when Biden dropped out in July, while Trump -- twice impeached while president -- has since ridden out two assassination attempts and a criminal conviction.

- 'Super excited' -

Casting a ballot in Arizona, Trump backer Camille Kroskey, 62, said she was voting in person due to concerns about voting fraud.

"I want to make sure I drop my ballot where it's going to actually land somewhere," she told AFP.

Harris will hold her watch party later at Howard University in Washington, a historically Black college that she attended as a student.

"I'm a black woman. I'm an American. I'm super excited about the possibility of her becoming president," a tearful Camille Franklin, who also went to the college, told AFP.

Trump has vowed an unprecedented deportation campaign of millions of undocumented immigrants, in a campaign full of dark rhetoric.

Harris has hammered home her opposition to Trump-backed abortion bans -- a vote-winning position with women.

The election was meanwhile being watched closely around the world including in the war zones of Ukraine and the Middle East, anxious to see how the next Oval Office occupant deals with the conflicts.