The Supreme Court on Wednesday acquitted former prime minister Somchai Wongsawat and three other defendants of abuse of authority in the fatal dispersal of PAD protesters in 2008.
The yellow-shirt People's Alliance for Democracy (PAD) consequently called on the national anti-graft body to appeal against the ruling.
Wednesday's ruling by the Supreme Court's Criminal Division for Holders of Political Positions cleared then prime minister Somchai, 70, his deputy prime minister Chavalit Yongchaiyudh, 85, and then national police chief Pol Gen Patcharawat Wongsuwon, 68, of dereliction of duty.
It also cleared then-metropolitan police commander Suchart Mueankaew, 66, of malfeasance.
Mr Somchai is the former brother in law of fugitive former prime minister Thaksin Shinawatra and Pol Gen Patcharawat is the younger brother of Deputy Prime Minister Gen Prawit Wongsuwon.
The charges were brought against them in 2015 by the National Anti-Corruption Commission (NACC), which served as the plaintiff in the criminal case.
On Oct 7, 2008, PAD members blockaded the parliament building after moving there from their occupation of Government House.
The demonstration intended to prevent Mr Somchai from delivering his policy announcement after he succeeded Samak Sundaravej, who was forced to resign the premiership.
Dispersal of the protesters left two dead and 471 injured.
The court in its ruling said the defendants had performed their duties as stated in the constitution in clearing a way for cabinet members and MPs to leave parliament on that fateful day.
It added that the protest did not fit the definition of a peaceful gathering, which is protected under the constitution.
According to the court, authorities found ping-pong bombs left at the scene.
Protesters also carried weapons during clashes with police officers, and used slingshots to shoot screws and marbles at the authorities.
The court said police had strictly followed a security plan to employ measures, going from soft to hard, to deal with the crowd.
Additionally, the defendants could not have foreseen that the tear gas used against the protesters would harm them, the court said.
According to an informed source, the judges voted eight to one to acquit the four defendants.
"I would like to thank the court for the ruling. I am grateful that justice still exists," said Mr Somchai after the ruling.
Veera Somkwamkid, a key yellow-shirt figure, called on the NACC to appeal the ruling as it is allowed under the new constitution.
Should the NACC decide not to do anything about the ruling, PAD supporters will probably discuss what they should do to win "justice", he said.
"It is a serious cause for concern whether today's ruling will set a benchmark for authorities who may take for granted [that they can do the same to other protesters in the future].
If some protesters bring weapons with them without the protest leaders knowing, it can be interpreted as the entire gathering not being peaceful. That is not fair towards the rest of the people at the same gathering," said Suriyasai Katasila, another former PAD leader.
Mr Suriyasai said he will have to study the ruling in detail before he can decide whether there are parts of it he could argue against or if there is any way to seek an appeal against it.
Udom Rathamarit, a member of the Constitution Drafting Committee, said the NACC is the plaintiff in this case, so it can exercise its right under Section 195 of the 2017 Constitution to appeal against the ruling within 30 days.
"It depends entirely on the NACC and whether it will try to find sound arguments before it actually appeals against the ruling or if it will simply opt to use the right allowed under the new charter to appeal," said Mr Udom.
Pol Gen Watcharapol Prasarnrajakit, the NACC chairman, said the NACC will meet today to discuss the court's ruling and what to do about it.
However, more in-depth discussions will later be conducted before the NACC decides on whether or not it will appeal against the ruling.
PAD spokesman Parnthep Pourpongpan, meanwhile, said that the PAD itself was denied the chance of being involved in the court battle in this criminal case, nor did its lawyers have the opportunity to take part in the fight for justice for the PAD.