"Your folks killed my dad. How could you do this? Now, it's your turn to beware," Terdsak shouted angrily before trying to punch Dr Sant Hathirat and Dr Weng Tochirakarn.
The incident erupted at Parliament House as Sant, who chairs the Democracy Federation, and Weng from the National United Front of Democracy Against Dictatorship, were reading a media statement urging people to fight against an attempt to stage a coup.
Sitting alongside Sant and Weng were Government chief whip Wittaya Buranasiri and People Power Party MP Nisit Sintuprai, chair of the House committee on parliamentary affairs
Somchai has stayed in Chiang Mai since he returned from Peru on Wednesday.
Many bloody confrontations between progovernment supporters and antigovernment People's Alliance for Democracy (PAD) protesters flared up this week. In one of these confrontations, Terdsak's father was killed in Chiang Mai.
Terdsak is described as a key supporter of PAD in the North. Security guards subdued him and he was sent to a press room.
"I don't take sides with any group. What I am doing is to protect the monarchy," Terdsak said yesterday.
He entered Parliament House to submit a petition to Senator Somchai Sawaengkarn, who chairs the Senate committee on human rights.
Somchai signed documents allowing Terdsak's entry.
According to Terdsak's petition, the progovernment crowds physically attacked him 15 times before killing his father.
"Please investigate these cases. The killing took place in the presence of policemen," Terdsak added.
Terdsak was later sent to Dusit police station for causing havoc in Parliament House.