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US President Donald Trump alleged on Wednesday that there had been "surprise ballot dumps" in states where he had been leading Democrat Joe Biden in the race for the White House.

"Last night I was leading, often solidly, in many key States, in almost all instances Democrat run & controlled," Trump tweeted. "Then, one by one, they started to magically disappear as surprise ballot dumps were counted."

Trump did not offer any evidence for his allegation of "ballot dumps" and there have been no reports of any irregularities.

Twitter has labeled Trump comments on ballot irregularities as "misleading".

The leads in numerous states have shifted back-and-forth between the candidates as votes are counted.

"How come every time they count Mail-In ballot dumps they are so devastating in their percentage and power of destruction?" he tweeted on Wednesday.

The usage of mail-in ballots soared this election amid the coronavirus pandemic and the US Elections Project said a record 65.2 million Americans voted by mail.

Result unclear

Americans woke up on Wednesday not knowing who the next US president would be as votes were still being counted in six key states that could swing the bitterly contested election to Republican incumbent Donald Trump or Democrat Joe Biden.

Fueling fears of a constitutional crisis, Trump prematurely declared victory overnight and threatened to demand the intervention of the Supreme Court to stop vote-counting but it continued nonetheless.

"We did win this election," the 74-year-old president told cheering supporters in the East Room of the White House before the final vote tallies were complete. "This is a fraud on the American public."

The Biden campaign slammed Trump's victory claim as "outrageous, unprecedented, and incorrect" and a "naked effort to take away the democratic rights of American citizens."

"The counting will not stop. It will continue until every duly cast vote is counted," it said. "If the president makes good on his threat to go to court to try to prevent the proper tabulation of votes, we have legal teams standing by ready to deploy to resist that effort."

The outcome appeared to hinge on the results from six states where a winner has not yet been declared - Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

As of 9 AM (1400 GMT), Biden had a slim lead in Michigan, Nevada and Wisconsin while Trump was ahead in Georgia, North Carolina and Pennsylvania.

But state election officials cautioned that with tens of thousands of ballots outstanding in some states - millions in others - the races could shift.

A deluge of mail-in ballots because of the coronavirus pandemic has slowed down the vote count in some states this year, several of which only began counting mail-in ballots on Tuesday.

Michigan officials, for example, said they hoped to have all of the votes counted by the end of the day while in Pennsylvania officials said it could be several more days.

"If everything keeps up, we'll have the total results in the next couple of days," Al Schmidt, the City Commissioner of Philadelphia, the largest city in Pennsylvania, told CNN.

"But Pennsylvania allows votes to be received and counted up until Friday and three days after the election.

"So we can't count what we don't yet have."

Record turnout

The US Elections Project estimated total turnout at a record 160 million voters including more than 101.1 early voters, 65.2 million of whom voted by mail.

In an election that took cast under the shadow of the coronavirus pandemic that has claimed more than 230,000 lives in the United States, Trump appeared to have avoided a Democratic wave predicted by some polls.

But it remained unclear Wednesday morning which candidate would capture the 270 votes needed for victory in the Electoral College that determines the winner of the presidential race.

Trump took the podium at the White House after 2 AM and declared that he would go to the Supreme Court because "we want all voting to stop".

Voting had already ended by the time Trump began speaking and he appeared to be calling for the nation's highest court to stop vote counting.

Trump has railed for months against mail-in ballots, charging without evidence they could be fraudulent.

Biden had earlier warned that vote counting would take a while as he greeted his own backers at a socially distanced rally in his home state of Delaware.

"We believe we're on track to win this election," the 77-year-old former vice president and senator from Delaware said. "Keep the faith, guys, we're going to win this."

Trump for the past four years has often been quick to say he is being treated unfairly but even a few of his fellow Republicans voiced unease at his dramatic intervention.

"Stop. Full stop. The votes will be counted and you will either win or lose. And America will accept that. Patience is a virtue," tweeted Adam Kinzinger, a Republican congressman who won reelection.

"I disagree with what he did tonight," said former New Jersey governor Chris Christie, who helped Trump prepare for his first debate against Biden.

"There's just no basis to make that argument tonight," Christie told ABC News. "There just isn't."

'Constitutional crisis'?

Foreign countries also sounded the alarm, with German Defence Minister Annegret Kramp-Karrenbauer warning Trump could create a "constitutional crisis."

Biden is the first Democrat in 24 years to win Arizona, seizing on the southwestern state's changing demographics and the popularity of astronaut Mark Kelly, who won a Senate seat held by a Republican.

But no other states immediately flipped and Trump won an early prize in Florida, where his hard line against Latin American leftists helped him make inroads among Cuban Americans.

Democratic hopes fizzled of turning around Texas, a Republican bastion indispensable for Trump, and Ohio.

Biden, as expected, comfortably won the biggest prize of all, California, as well as New York and easily kept Minnesota and New Hampshire, two states where Hillary Clinton in 2016 had only eked out victories over Trump.

Attention again turned to three states that elected Trump four years ago - Michigan, Pennsylvania and Wisconsin - with ballots still waiting to be counted from the Democratic stronghold cities of Detroit, Philadelphia and Milwaukee.

Biden said he was feeling "real good" about Michigan and Wisconsin and voiced confidence about Pennsylvania, where he was born.

Read more: Five big takeaways from the US election so far

Biden said he was also competitive in Georgia - a state that until recently had not appeared to be in play - as election workers in its largest city Atlanta halted counting for the night after a pipe burst.

Pundits had been warning for weeks that this year's election results would take time - and voiced fears Trump would cause chaos or even violence by questioning the process.

While there were no immediate reports of unrest, stores have been boarded up throughout the capital Washington.