"We will never give up, we will never concede," Trump told the attendees outside the White House, who cheered him on one hour before Congress was set to begin the process of confirming Biden's victory in the Electoral College.
"We will stop the steal!" Trump said when taking the stage, after the crowd waited for his appearance while listening to pop songs like "Macho Man," "Don't Stop Believing" and "Tiny Dancer."
The lame-duck Republican also reiterated a call on his own vice president, Mike Pence, to refuse to accept the validity of Electoral College ballots for Biden from several battleground states.
Experts say Pence has no such power when he presides over the joint session of Congress as it confirms state election results certifications on Wednesday afternoon.
"I hope Mike is going to do the right thing," Trump said. "If Mike Pence does the right thing, we win the election."
"He has the absolute right to do it."
Pence later Wednesday refused Trump's demand that he try to block the confirmation of Biden as the next president. Pence said in a letter that he did not believe, as Trump has claimed, that a vice president has the unilateral power to reject Electoral College votes for a candidate.
More than a dozen Republican senators and up to 100 or more House members are expected to challenge the results from some states, but that effort is not expected to thwart Biden's victory, because both the Senate and House are on track to confirm his wins in those states.
Trump, as he has for two months, claimed that he won the popular vote outright, when he actually lost by more than 7 million popular votes to the former Democratic vice president.
"We won it by a landslide," he claimed. "This was no close election."
Trump also claimed that "they cheated like hell" in Georgia's two special elections for the Senate on Tuesday, where Democratic challenger Raphael Warnock has been projected the winner over Republican Sen. Kelly Loeffler, and where another Democrat, Jon Ossoff, was leading Republican David Perdue in ballot tallies.
Trump and his allies claim, without any evidence, that there was widespread ballot fraud in multiple states, which in turn led to those states allowing their Electoral College votes to be awarded to Biden. The Democrat won 306 electoral votes, 36 more than he needs to win the White House.
Trump had won by the exact same margin in 2016 over Hillary Clinton.
"They rigged an election. They rigged it like never before," Trump said Wednesday.
However, no court has invalidated any ballot for Biden on a claim by the Trump campaign or its supporters that fraud was committed.
Trump-related legal efforts challenging Biden's victory, which has included a request that the U.S. Supreme Court step into the issue, have been defeated or withdrawn across the board.
The president bemoaned the Supreme Court, which has three Trump-appointed justices, saying the high court "loves to rule against me."
Trump's rambling remarks Wednesday soon devolved into him asking "Where's Hunter?"
He had hoped claims he raised about Biden's son Hunter Biden during the election would help win him win a second term.