A popular activist YouTuber known for his First Amendment video stunts was arrested Thursday after he showed up uninvited to the Suffolk County PBA’s headquarters, union leaders told The Post.

Sean Reyes, aka The Long Island Auditor, was arrested for trespassing at the Police Benevolent Association building in Brentwood as he confronted union members on video over reports that cops who admitted to misconduct were allowed to stay on the force until their pensions kicked in, PBA President Lou Civello said.

“He was threatening us — he threatened to come to my home,” Civello claimed in a brief interview.

An official spokesperson for the department couldn’t immediately confirm the charges.

The activist, who is known to film in public buildings in a supposed test of First Amendment rights, has 1.03 million YouTube subscribers.

Reyes was seen at the Suffolk County Board of Legislatures on Tuesday before his arrest, confronting elected officials on the reporting. He also claimed that he found a Suffolk cop snooping in his backyard.