US President Donald Trump erupted on Thursday in a tirade of unsubstantiated claims that he has been cheated out of winning the US election as vote counting across battleground states showed Democrat Joe Biden steadily closing in on victory.

“They are trying to steal the election,” an increasingly isolated Trump said in an extraordinary statement at the White House two days after polls closed.

Providing no evidence and taking no questions afterward from reporters, Trump spent nearly 17 minutes making the kind of incendiary statements about the country's democratic process that have never been heard before from a US president.

According to Trump, Democrats were using “illegal votes” to “steal the election from us”.

“If you count the legal votes, I easily win,” he claimed. “They're trying to rig an election. And we can't let that happen.”

Beyond the rhetoric, Trump's complaints were specifically targeting the integrity of the huge number of ballots mailed in, rather than cast in person on Election Day.

The big shift to postal ballots this year reflected the desire of voters to avoid risking exposure to Covid-19 in crowded polling stations during a pandemic that has already killed close to 235,000 Americans.

However, because Trump often denied the seriousness of the virus and told his supporters not to support mail-in ballots, far fewer Republicans took advantage of the option, compared with Democrats.

Biden closes in

Several major US television networks cut away from live coverage of Trump's event and there were signs of cracks in Republican support.

Representative Will Hurd called Trump's call to stop vote-counting “dangerous and wrong,” saying it “undermines the very foundation this nation was built upon”.

And conservative Rupert Murdoch's powerful media empire appeared to shift tone, with his New York Post calling Trump's fraud allegations “baseless” and Fox News refusing Republican pressure to rescind its projection that Trump would lose Arizona, vital for his chances.

Trump's rant came as returns from still undeclared states around the country showed Biden trending toward victory.

Biden, 77, was just one or at most two battleground states away from securing the majority to take the White House. Trump, 74, needed an increasingly unlikely combination of wins in multiple states to stay in power.

Biden, who has promised to heal a country bruised by Trump's extraordinarily polarising four years in power, appealed for “people to stay calm”.

“We have no doubt that when the count is finished, Senator (Kamala) Harris and I will be declared the winners,” he said in comments to reporters in his hometown of Wilmington.

“The process is working,” he said. “The count is being completed. And we will know soon.”

All about Pennsylvania?

In Georgia, a generally Republican state, Trump had a razor thin and fast vanishing lead of fewer than 1,900 votes. In Arizona and Nevada, Biden held on to slim leads. If Biden wins both those states he would also win the presidency.

But the biggest piece of the puzzle was Pennsylvania, where Trump's early lead was again steadily draining away, as election officials homed in on processing mail-in ballots, which are more typically cast by Biden supporters.

The Democratic hopeful currently has 253 of the 538 electoral college votes divvied up between the country's 50 states. He has 264 with the inclusion of Arizona, which Fox News and the Associated Press have called in his favor, but other major organisations have not.

If Biden took Pennsylvania, he would grab 20 more electoral college votes, thereby instantly topping the necessary 270 for overall victory.

Latest results showed Trump's lead in the state had shrunk to around 42,000 votes, with most votes yet to be counted coming from Democratic stronghold Philadelphia.

Protests across country

Trump's campaign continued to insist that the president has a way to win, citing pockets of Republican support yet to be counted.

But Trump's overwhelming focus was on claiming, without evidence, that he was a victim of mass fraud.

Trump prematurely declared victory on Wednesday and threatened to seek Supreme Court intervention to stop vote-counting but it has continued nonetheless.

Since then, his team fanned out across the battleground states challenging the results in court and his supporters converged outside election offices in several cities.

In Las Vegas, Trump backers wearing red “Make America Great Again” hats demanded to see ballots being processed.

Brando Madrigal said he wanted to verify that the votes are “not coming from the people who died with Covid, people who are out of state, people who don't have the ability to vote because they don't have the papers”.

But while Trump was demanding that counting be halted in Georgia and Pennsylvania — where he is leading — his supporters and campaign insisted that it continue in Arizona and Nevada, where he is trailing.

Bob Bauer, a lawyer for the Biden campaign, dismissed the slew of lawsuits as “meritless”.

“All of this is intended to create a large cloud,” Bauer said. “But it's not a very thick cloud. We see through it. So do the courts and so do election officials.“