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 Weerawit Rungruangsiripol was waiting for him outside the complaints centre when he arrived to ask police to take action against the popular comic.
Mr Weerawit was seen launching his attack, punching, grabbing and kicking at Mr Srisuwan, who was wearing a red shirt, before police and onlookers intervened.
Mr Weerawit 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 show 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 a 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 stirring comments.
He said he did not ask for police action against the comedian, 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 one of the scenes in 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 is widely known as "Phi Sri". Phi means brother in Thai and Sri is the first syllable of his first name.
Mr Srisuwan told reporters after coming out of the complaints centre that his face was only slightly hurt by the punch.
"They call themselves a democratic group but they use violence against other people. That is a contradiction," he said.
Weerawich Runruangsiriphol attacks activist Srisuwan Janya (red shirt) outside the complaints centre of the Central Investigation Bureau on Tuesday, (Screen capture from video).