When Brad Lander invited reporters to federal immigration court in Lower Manhattan on Tuesday, he might have predicted some, but not all, of what would follow.

Mr. Lander, the New York City comptroller who is running for mayor, said his prime goal was to help escort immigrants from the building, acting as a physical buffer to dissuade immigration agents from interceding.

He said that getting arrested “was certainly not my plan.”

But Mr. Lander was arrested, roughly handled and handcuffed by federal agents, with the confrontation recorded and seen by millions. He was held for several hours before the authorities released him without filing criminal charges.

And when he emerged from custody, he was met by a cheering crowd of hundreds of people, including Gov. Kathy Hochul.

His arrest put him at the center of a national debate over immigration and due process following the similar treatment of other Democrats, including Senator Alex Padilla of California and Mayor Ras Baraka of Newark.

It was a rare moment of unity on the mayoral campaign trail, with several candidates criticizing his arrest and calling for his release, and a moment in the spotlight for Mr. Lander, who has been running a distant third in the polls.