Trump, who has spent much of his time since the election in a failed effort to reverse his defeat, falsely said he won the November 3rd contest as he spoke on an outdoor stage framing the White House, which Biden is due to take over in two weeks.
Crowds gathered at the "Save America March" wore Trump-approved red baseball caps and cheered as Trump repeated the groundless conspiracy theories that have consumed his final days in office.
"You don't concede when there's theft involved," Trump said after taking the stage. "Our country has had enough and we will not take it any more."
He ramped up pressure on Vice President Mike Pence to block congressional certification of the November election results on Wednesday, saying, "If Mike Pence does the right thing, we win the election."
He aired his familiar grievances with the news media and Hillary Clinton, his former Democratic rival from the 2016 election, and delighted the crowd by calling Democratic victories the product of what he called "explosions of bullshit.""
"Bullshit! Bullshit! Bullshit!" the crowd chanted in reply.
Weeks have passed since the states completed certifying that Biden, a Democrat, won the election by 306 Electoral College votes to Trump's 232, and Trump's extraordinary challenges to Biden's victory have floundered in courts across the country.
Congress was due to confirm Biden's victory on Wednesday. Many of Trump's fellow Republicans have promised a stonewalling effort that could stretch proceedings past midnight but was almost certain to fail.
DONALD TRUMP: We will never give up. We will never concede. It doesn't happen. You don't concede when there's theft involved.
And I hope Mike is going to do the right thing. I hope so. I hope so. Because if Mike Pence does the right thing, we win the election.
All Vice President Pence has to do is send it back to the States to recertify, and we become president, and you are the happiest people.
That election, our election, was over at 10 o'clock in the evening. We're leading Pennsylvania, Michigan, Georgia, by hundreds of thousands of votes. And then late in the evening, or early in the morning, boom, these explosions of bullshit. And all of a sudden, all of a sudden, it started to happen.
[CROWD CHANTING "BULL SHIT"]
Now it is up to Congress to confront this egregious assault on our democracy. And after this, we're going to walk down-- and I'll be there with you-- we're going to walk down, we're going to walk down-- any one you want, but I think right here-- we're going to walk down to the Capitol, and we're going to cheer on our brave senators and congressmen and women. And we're probably not going to be cheering so much for some of them. Because you'll never take back our country with weakness. You have to show strength and you have to be strong.