“He won because the Election was Rigged,” the Republican president wrote on Twitter, repeating unsubstantiated claims of election fraud.
About an hour later he said he was not conceding the 3 November vote.
“He won because the Election was Rigged,” Trump said in one Sunday morning tweet. In another, he stood by his false belief he may be able to win the election and refused to concede, writing, “I concede NOTHING! We have a long way to go.”
He has launched a slew of lawsuits in key states, but has not provided any evidence to back his claims of fraud.
All the lawsuits have so far been unsuccessful.
Most news networks declared Biden the victor more than a week ago. Since the call was made, Biden has given a victory speech and gotten his transition team up and running. But Trump, who had questioned the validity of the election before votes had even been cast, is continuing to dig in his heels on unsupported legal challenges lodged by his allies over vote counts in several states and asserting that the fight has only begun.
On Friday, election officials said the vote was the “most secure in American history” and there was “no evidence that any voting system deleted or lost votes, changed votes or was in any way compromised”.
The Democrat has 306 votes in the electoral college – the system the US uses to choose its president – which far exceeds the 270 threshold to win. Any recounts or legal challenges are not expected to overturn the overall result.
Mr Biden’s lead in the popular vote has also surpassed five million.
Nevertheless, Mr Trump had refused to acknowledge Mr Biden’s victory until – apparently – now.
In a news conference on Friday, Mr Trump said “who knows” which administration would be in power in the future.
His refusal to concede has heightened concerns about the US government’s ability, going forward, to tackle growing Covid-19 infection rates.
Some Biden supporters consider the president’s comments amounted to a form of concession, although it was far from unqualified.
“He won”, began President Trump’s initial tweet, before descending into his customary slew of unsubstantiated allegations of electoral fraud.
Mr Biden won, continued the president, because there were “no vote watchers or observers allowed”, repeating his claims of a conspiracy led by the left and the media – while still presenting no evidence.
Indeed, within a couple of hours, he contradicted his “He won” comment with another tweet, which read: “He only won in the eyes of the fake news media, I concede nothing.”
In essence, that is about as far as President Trump is likely to go. Even if he eventually acknowledges he won’t be president from 20 January, he will probably never relinquish his unsubstantiated claims that he was beaten in a fraudulent vote.
Twitter added warnings to Mr Trump’s latest allegations of wrongdoing on Sunday, saying: “This claim of election fraud is disputed.”
The Trump campaign has launched a series of lawsuits saying Republican poll watchers were wrongly denied access to observe the counting of ballots in key battleground states.
Election officials have denied this and insisted rules were followed. Most of the lawsuits have been dismissed over lack of evidence.
However, thousands of Mr Trump’s supporters protested in Washington DC on Saturday to back his appeals.
Flag-carrying demonstrators were joined by members of far-right groups including the Proud Boys, some wearing helmets and bullet-proof vests.
The largely peaceful demonstration saw some violence later in the evening, as Trump supporters and counter-protesters clashed in several skirmishes.
Officials said 20 people had been arrested on a variety of charges, including assault and weapons possession. One stabbing was reported. Two police officers were also injured.
Since Biden declared victory last week, the President has spent little time in front of cameras and essentially no time answering questions from reporters he doesn’t perceive as allies. But on Twitter this weekend, the President launched baseless conspiracy theories and false statements to his nearly 90 million followers online.
Trump has repeatedly claimed without evidence that his chance at a second term in office was stolen from him with corrupt votes. A previous fact check by CNN of claims by a handful of prominent Republicans, including members of Trump’s family and supporters like former House Speaker Newt Gingrich and former acting Director of National Intelligence Richard Grenell, of dead people voting in Michigan fell apart under scrutiny.