The People's Alliance for Democracy (PAD) has defied a government warning to comply with the law regulating public assembly, arguing the group does not hold political rallies to pressure the government.
Meanwhile, yellow-shirt leaders have pressured the National Anti-Corruption Commission (NACC) to appeal the Supreme Court's acquittal of former prime minister Somchai Wongsawat and three others for abusing their authority in the deadly dispersal of protesters in 2008.
The three figures are then deputy prime minister Chavalit Yongchaiyudh, now 85, former national police chief Patcharawat Wongsuwon, 68, and ex-metropolitan police commander Suchart Mueankaew, 66.
The decision emerged from a meeting yesterday morning of key PAD leaders including Pibhop Dhongchai, Suriyasai Katasila, Veera Somkwamkid, Parnthep Pourpongpan and other PAD members to discuss the court's ruling.
In an interview with the Bangkok Post, former PAD coordinator Suriyasai Katasila said the group gathered on Friday to compile facts and arguments to present to the NACC for use in appealing the court's decision. He stressed it was not a "political gathering".
Mr Suriyasai insisted the PAD did not make any political move to pressure the government, adding that Deputy Prime Minister Prawit Wongsuwon, who oversees national security, should be more discreet and avoid being too heavy-handed in enforcing the public gathering law.
Gen Prawit issued another warning that the PAD must strictly follow the order by the National Council for Peace and Order (NCPO) banning political gatherings that involve more than five people.
According to the new law on public gatherings, the group must first obtain permission from the authorities if it plans to hold any kind of public assembly.
NCPO spokesman Winthai Suvaree said he was not sure if yesterday's media briefing held by the PAD would risk breaching the ban. A legal team will look into the matter, Col Winthai said.
The PAD is dissatisfied with a ruling by the Supreme Court's Criminal Division for Holders of Political Positions that cleared Mr Somchai, 70, and three other defendants of any wrongdoing in the crackdown on PAD protesters in 2008.
The group is expected to plan a response to the ruling but it is not clear whether that would involve street protests given the regime's opposition to them.
Mr Suriyasai said the case has affected hundreds of people, with three deaths and almost 500 injuries, making the participation of their relatives much needed.
They feel the fight in the case is not yet over and as stakeholders they must be allowed to exercise their right to seek an appeal, Mr Suriyasai said.
"Deep down, I am confident certain facts [of the case] have been overlooked or that there is still new information which will cause the trial process to continue. This will have to be proven further and this case should be useful to society.
"We don't want to send anyone to jail. We want to set a decent precedent -- a precedent created by people who made sacrifices to fight for the ouster of an illegitimate government," Mr Suriyasai said.
Reading out the group's statement, Mr Parnthep said former key PAD figures, victims of the crackdown and their relatives respected the court's judgement, but disagreed with it.
The ruling overlooked various facts, and an appeal should be submitted to a plenary session of Supreme Court judges, he said.
"We respect the Supreme Court's decision but disagree with it entirely on the grounds that the ruling is not just," Mr Parnthep said.
"We see there is still an opportunity to bring this issue to justice by requesting an appeal through the NACC," he said.
The decision also conflicts with the views of the Central Administrative Court, the NACC and the National Human Rights Commission, which earlier stated the PAD rally was peaceful and weapons-free, and therefore protected under the constitution, Mr Parnthep said.
Prime Minister Prayut Chan-o-cha said the NACC will decide whether or not to appeal.
A source in the NACC said the commission will need to study key pieces of information before making a decision.
The commission will study the individual ruling of the minority judge who felt the defendants should have been found guilty. It will also look at previous Supreme Court rulings on similar cases as well as information supplied by the PAD supporting its stance.
To put forward an appeal to the Supreme Court, new evidence and facts must be produced to convince the judges the case is worthy of reconsideration.
The NACC has been given 30 days to decide on whether to appeal.