Marchers close to the White House greeted Trump's brief appearance with wild cheers, waving and whistles, holding signs and flags saying "Best prez ever," "Stop the steal" and "Trump 2020: Keep America Great."
Several thousand people had gathered by noon on the city's Freedom Plaza, with more still arriving from all sides, waving flags and shouting "Four More Years" in a festive atmosphere reminiscent of a Trump rally.
With right-wing militia group the Proud Boys also among those rallying, a large security presence was deployed in the capital to prevent clashes with anti-Trump events scheduled outside the Supreme Court.
The latest tallies gave Trump's Democratic challenger Joe Biden a solid final win in the state-by-state Electoral College that decides the presidency, with 306 votes against Trump's 232. Two hundred seventy votes are required for election.
But Margarita Urtubey, 49, a horse breeder who flew in from Miami with her sister, told AFP the election was "so corrupt," adding "Trump won by a landslide. We are here to march for the 'stop the steal' of this election, to make our voice heard."