The United Front for Democracy against Dictatorship (UDD) stands by the government and is ready to come out to protect it, says its co-leader Nattawut Saikuar.
Mr Nattawut, who is also the deputy commerce minister, claimed that overthrowing the government was the true motive behind the anti-amnesty rally being led by the Democrat Party.
The opposition party, he said, was following the example of the movement of the yellow-shirt People's Alliance for Democracy (PAD) when it worked to unseat the government of then-prime minister Thaksin Shinawatra.
The yellow shirts launched a street rally against Thaksin before he was toppled by the military coup in 2006.
"Many UDD members, including me, have already revealed our stances toward the ruling party's decision to pass the amended amnesty bill," Mr Nattawut said on Tuesday.
"Taking further steps against the move [by Pheu Thai MPs] would only create conflicts within the group, hence benefitting the opposition party.
"Therefore the group will remain uninvolved. But the group will keep a close watch on the current situation."
Mr Nattawut was one of the four Pheu Thai MPs who abstained from voting on the legislation on its second and third readings. The others included Worachai Hema, Weng Tojirakarn and Khattiya Sawatdiphol, all of whom are UDD members.
Three of them including Mr Nattawut face terrorism charges arising from their participation in the 2010 red-shirt rallies.
Many red-shirt supporters oppose the legislation because it would grant amnesty to former prime minister Abhisit Vejjajiva and his second-in-command Suthep Thaugsuban, both of whom were indicted last month by the Office of the Attorney-General for authorising the crackdown three years ago in which many red shirts were slain.
However, Panwadee Tantisirin, a leading member of the red shirts in Khon Kaen, said top UDD members from 26 districts and neighbouring provinces had gathered to show their support for the draft amnesty bill.
Ms Panwadee advised the government to clarify to the public the details of the legislation, saying that protests against the bill stem from the public misinterpreting it.
Mayuree Saetasai, co-leader of the red shirts in Ayutthaya, also backed the amnesty bill, saying the bill would do more good than harm.
Ms Mayuree said the Democrats' move to include the public in their political game was a big mistake because the majority of the people in the country are red-shirt supporters.