Mr. Segal then outlined levels, or “degrees,” the Proud Boys recognize. Third-degree members, for instance, tattoo the name of the group on their bodies, and under rules that were in effect in October 2018 but have since been revised, members reached the fourth, or highest, degree by getting in a physical confrontation or getting arrested.

Although the defendants are the ones who stand formally accused of breaking the law, Antifa is both nowhere and everywhere in the trial, physically absent from the courtroom but invoked regularly by the defense.

Mr. Hare took the stand on Thursday afternoon, telling jurors that he had joined the Proud Boys in early 2017, seeing it as “a group of blue collar men who love America” and like to drink together.

He also said he considered Antifa to be a “violent, militant, anarchist group” and said he was afraid when protesters showed up outside the Metropolitan Republican Club during Mr. McInnes’s speech.

Mr. Hare, an Amtrak employee, said that he had not punched one of the protesters at the start of the clash on 82nd Street as Detective Mays testified, but instead had tried to remove a mask that person was wearing. Later in the melee, he said, he kicked and punched others to protect himself.

He first spotted the small group of masked protesters when they were about 70 feet away, Mr. Hare said. He moved toward them, he claimed, because he believed they posed a threat. “They were attacking us,” he said. “They were going to hurt us.”

Under cross-examination, which will continue next week, Mr. Hare acknowledged that he had become a fourth-degree Proud Boy the week before the conflict on the Upper East Side. He had traveled to Providence, R.I., where he helped provide security for a right-wing rally and at one point punched someone in the face.