The first of the seven key battlegrounds -- North Carolina -- went to Trump, confirming his growing momentum.

Democratic vice president Harris appeared to be underperforming in other key areas compared to the Republican former president.

Harris's camp said the race was now "razor-thin" and that her "clearest path" to victory was through the so-called Blue Wall swing states of Michigan, Pennsylvania and Wisconsin.

Harris's campaign director Jen O'Malley Dillon insisted in an mail to campaign staff, obtained by AFP, that "we feel good about what we’re seeing" in the Blue Wall.

Trump spokesman Jason Miller said the mood in the Republican's camp in Florida was "positive."

The US dollar surged and bitcoin hit a record high while most equity markets advanced as traders bet on a victory for Trump as the results rolled in.

Trump's early wins included the predictably Republican Florida and Texas while Harris won California, giving Trump 227 electoral votes and Harris 172. The magic number to win the presidency is 270.

Results from the other closely-fought main swing states were all still pending, but Trump was maintaining a lead in Georgia.

Republicans also flipped two Senate seats, in a major blow to Democrats as they fight tooth and nail to maintain their advantage in the chamber.

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

They were deciding whether to either hand a historic comeback to Trump and his right-wing "America First" agenda or make Harris the first woman in the world's most powerful job.

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

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."

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 had 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.

A final overall result in the presidential race could still take hours -- or even days.

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.