WASHINGTON — For years, they have lurked in the web’s dark corners, masking themselves with cartoon images and writing screeds about the demise of white culture under ominous pseudonyms. But on Saturday, in the wake of Donald J. Trump’s surprising election victory, hundreds of his extremist supporters converged on the capital to herald a moment of political ascendance that many had thought to be far away.

In the bowels of the Ronald Reagan Building and International Trade Center, three blocks from the White House, members of the so-called alt-right movement gathered for what they had supposed would be an autopsy to plot their grim future under a Clinton administration. Instead, they celebrated the unexpected march of their white nationalist ideas toward the mainstream, portraying Mr. Trump’s win as validation that the tide had turned in their fight to preserve white culture.

“It’s been an awakening,” Richard B. Spencer, who is credited with coining the term alt-right, said at the gathering on Saturday. “This is what a successful movement looks like.”

The movement has been critical of politicians of all stripes for promoting diversity, immigration and perceived political correctness. Its critics call it a rebranded version of the Ku Klux Klan, promoting anti-Semitism, violence and suppression of minorities.