Serial petitioner Srisuwan Janya was assaulted, but not seriously injured, when he arrived to register a complaint against comedian Udom "Nose" Taepanich with police at the Central Investigation Bureau on Tuesday.
A group led by red-shirt leader Weerawich Runruangsiriphol was waiting for him outside the complaints centre when he arrived to ask police to take action against the popular comic.
Mr Weerawich was seen launching an attack, punching, grabbing and kicking at Mr Srisuwan, who was wearing a pink shirt, before police and onlookers intervened.
Mr Weerawich told reporters that he had been waiting for this day to teach the secretary-general of the Association for the Protection of the Thai Constitution a lesson, ever since he threatened lawsuits against protesters planning rallies against the ruling of the Constitutional Court on the tenure of Prime Minister Prayut Chan-o-cha.
The red-shirt group outside the complaints centre also accused Mr Srisuwan of siding with the government.
The comedian angered supporters of Gen Prayut with several comments made in his latest stand-up special Deaw 13, which streamed on Netflix on Oct 11. One of them was: "Traffic is a bit congested because there is a mob there to oust him. Please forgive them. They are doing that for us."
That was apparently aimed at Gen Prayut.
Mr Srisuwan used this as the basis to register his complaint, accusing "Nose" of attempting to disrupt law and order in breach of the Criminal Code, and saying that national security could also be endangered by his inflammatory comments.
He said he did not wish for police action against the comedian just to please the government.
"My complaint today has nothing to do with the government. I have lodged complaints against the government whenever it did something wrong," he said.
Deputy Prime Minister Prawit Wongsuwon on Sunday brushed aside the seriousness of the issue, saying the show was just entertainment. "I think he's doing it for entertainment, and the audience should use their discretion."
In another part of Deaw 13. "Nose" also touched on Mr Srisuwan, although he did not mention him by his full name.
The comedian asked the stage crew to focus more spotlights on him, and then said: "Now I understand why 'Phi Sri' is hungry for the public spotlight."
Mr Srisuwan, who is widely known as "Phi Sri" ("older brother Sri)", told reporters as he left the complaints centre that he was only slightly hurt.
"They call themselves a democratic group, but they use violence against other people. That is a contradiction," he said.
He also filed a complaint with police against Mr Weerawich for assault.