US President Donald Trump has accused Democrats of voting "shenanigans" as the result from Tuesday's White House election hangs in the balance.
His Democratic challenger Joe Biden earlier appealed for calm as the nail-biting count drags on in five states.
While clinging to wafer-thin leads in Nevada and Arizona, Mr Biden has been chewing into the Republican president's edge in Pennsylvania and Georgia.
The cliff-hanger follows one of the bitterest campaigns in living memory.
The election is projected to have generated the highest turnout since 1900. Mr Biden has so far pulled in well over 73 million votes, the most ever for a US presidential candidate. Mr Trump has drawn almost 70 million, the second-highest tally in history.
What did Trump say?
In his first public remarks since appearing at the White House in the early hours of Wednesday, the president said: "If you count the legal votes, I easily win.
"If you count the illegal votes they can try to steal the election from us."
Beyond allegations of irregularities, the Trump campaign has not presented any evidence of election fraud.
Speaking from the White House on Thursday, the president added: "We were winning in all the key locations, by a lot actually, and then our numbers started getting miraculously whittled away in secret and they wouldn't allow legally permissible observers."
Mr Trump's critics have pointed out his lead is evaporating because he actively discouraged his supporters from voting by mail, while Mr Biden urged his voters to do so, and it is these postal ballots that are now being tallied in the key states.
The president added: "There's been a lot of shenanigans and we can't stand for that in our country."
Election analysts have argued that the president's claims of Democratic electoral corruption are undermined by the better-than-expected performance of his fellow Republicans in congressional races across the map.
What's the reaction?
Some Republicans who have in the past raised rare dissent against the president implicitly criticised his latest remarks.
Mitt Romney, the Utah senator who lost as the Republican presidential nominee in 2012, said: "The votes will be counted. If there are irregularities alleged, they will be investigated and ultimately resolved in the courts.
"Have faith in our democracy, in our Constitution, and in the American people."
Also without naming the president, Illinois congressman Adam Kinzinger said: "If you have legit concerns about fraud present evidence and take it to court. Stop spreading debunked misinformation... This is getting insane."
But Texas congressman Will Hurd, a Republican who did not seek re-election, was more forthright, calling Mr Trump's comments "dangerous" and "wrong".
And Maryland Governor Larry Hogan tweeted: "There is no defence for the President's comments tonight undermining our Democratic process."
However, Georgia Senator David Perdue tweeted that "every lawful vote cast should be counted, once", and then the president would win.
What did Biden say?
Earlier on Thursday, speaking from his campaign headquarters in Wilmington, Delaware, Mr Biden appealed for calm across the country.
In a brief televised address, the Democratic challenger again expressed confidence he would be declared the winner.
"Democracy is sometimes messy," he said. "It sometimes requires a little patience as well.
"But that patience has been rewarded now for more than 240 years, the system of governance that has been the envy of the world."
He added: "I asked everyone to stay calm. All people to stay calm. The process is working. The count is being completed. And we'll know very soon."
As results gradually trickle in, protests involving both sides have been held in major cities over the vote counting.