“We’ve had an enormous cultural and political impact,” said David Graeber, a professor at the London School of Economics who helped organize the Occupy protests and has been credited with coining its “we are the 99 percent” slogan. He said the movement had elevated income inequality to the top of the Democratic political agenda, despite not electing anyone or enacting any legislation.

But he said Mr. Trump’s victory had proved that anarchists’ diagnosis of society’s ills was correct.

“We tried to warn you, with Occupy,” Dr. Graeber said. “We understood that people were sick of the political system, which is fundamentally corrupt. People want something radically different.”

Mr. Trump’s tirades against trade deals, globalization and a Washington elite he views as corrupt mirror arguments that anarchists have been making for decades. But his claim that he alone can fix America’s problems flies in the face of anarchists’ conviction that only direct action by ordinary people can produce a fair system.

“Fascism fetishizes having a strong leader who is decisive and tells everyone what to do,” Mr. Laursen, the writer, said. “That’s what we are seeing with Trump.”

Fueled in part by Mr. Trump’s political success, violent clashes between the far right and far left erupted several times during the presidential campaign. In Anaheim, Calif., last February, three people were stabbed in a brawl after anti-fascists disrupted a Ku Klux Klan rally. And in Sacramento in June, at least five people were stabbed and eight wounded when hundreds of counterprotesters, including anti-fascists, clashed with skinheads at a rally.

But the confrontations seemed to shift into a new gear on the eve of Mr. Trump’s inauguration. On Jan. 19, anti-fascists tried to block the entrance to the “DeploraBall,” a party for Trump supporters. The next day, 230 people were arrested after anarchists dressed in black broke the windows of a bank with baseball bats and set a limousine on fire. (Mr. Spencer, the white nationalist, whose assailant was not arrested, was not the only person struck: A videographer was struck in the chest with a flagpole — he was unharmed — as he tried to interview marching anarchists about what the word “community” meant to them.)