Kyle Chapman expected he might find a fight. And he did — with a teenage girl.

The girl was waving an anti-fascist placard last week at a protest against Shariah law in Midtown Manhattan when a scuffle broke out and she knocked an older woman to the ground.

“Assaulting our people?” Mr. Chapman shouted as he reached across the barricades and ripped her sign apart. “Your days are numbered, Commie!” he called after her as the police escorted her away. “The American people are rising up against you!”

As the founder of a group of right-wing vigilantes called the Fraternal Order of Alt-Knights, Mr. Chapman, a 6-foot-2, 240-pound commercial diver, is part of a growing movement that experts on political extremism say has injected a new element of violence into street demonstrations across the country.

Part fight club, part Western-pride fraternity, the Alt-Knights and similar groups recruit battalions of mainly young white men for one-off confrontations with their ideological enemies — the black-clad left-wing militants who disrupted President Trump’s inauguration and have protested against the appearances of conservative speakers on college campuses.