"The simple fact is this election is far from over," Trump said in a statement released just minutes after NBC projected that Biden had become president-elect.
"Joe Biden has not been certified as the winner of any states, let alone any of the highly contested states headed for mandatory recounts, or states where our campaign has valid and legitimate legal challenges that could determine the ultimate victor," Trump claimed.
Trump vowed that as soon as Monday, his team will start "prosecuting our case in court to ensure election laws are fully upheld."
Biden earlier Saturday had led Trump in NBC's tally of projected Electoral College votes, 253 to 214. The former vice president was pushed over the top when NBC projected that Pennsylvania's 20 electoral votes would be awarded to him.
Trump had traveled to his private golf club in Sterling, Virginia, less than two hours before the race was called for his opponent.
Biden, in a statement released around noon Saturday, said he is "honored and humbled by the trust the American people have placed in me and in Vice President-elect [Kamala] Harris."
"In the face of unprecedented obstacles, a record number of Americans voted. Proving once again, that democracy beats deep in the heart of America," the president-elect said. "With the campaign over, it's time to put the anger and the harsh rhetoric behind us and come together as a nation."
But the Trump campaign disputes that the campaign has come to an end. The president and his surrogates have launched lawsuits in multiple key states, including Pennsylvania and Michigan, and have already signaled they plan to press for recounts in some close races.
Trump himself has spread unproven allegations of electoral fraud in key states, despite repeated pushback from election officials and judges in numerous court cases. As vote tallies in crucial states appeared to be favoring Biden in the days after Election Day, Trump even demanded that states stop counting ballots.
"Obviously he's not going to concede when at least 600,000 ballots are in question," Trump's attorney, former New York Mayor Rudy Giuliani, said at a press conference in Philadelphia on Saturday, without providing evidence.
Meanwhile, world leaders came out of the woodwork to congratulate Biden and Harris on winning the race against Trump and Vice President Mike Pence.
Canadian Prime Minister Justin Trudeau said in a tweet to Biden and Harris, "I'm really looking forward to working together and building on that with you both."
Scotland's First Minister Nicola Sturgeon tweeted, "Congratulations from to President-Elect Joe Biden and to history-making Vice President-Elect Kamala Harris."
United Kingdom Prime Minister Boris Johnson, long seen as one of Trump's closest allies on the world stage, also congratulated the projected president-elect.