Red-shirt lawyers will next week file a formal petition for the National Anti-Corruption Commission (NACC) to consider "new evidence" regarding the 2010 crackdown on protesters.
United Front for Democracy against Dictatorship (UDD) secretary-general Nattawut Saikuar said the petition will detail a request that it revive investigations against former prime minister Abhisit Vejjajiva and his ex-deputy Suthep Thaugsuban, who oversaw the 2010 crackdown against red-shirt protesters which left at least 91 dead in the capital and scores of injuries.
The move came after a recent Supreme Court ruling which rejected the case, saying the matter must go to the Supreme Court's Criminal Division for Holders of Political Positions via an investigation by the NACC instead of the Criminal Court.
The NACC under the old commissioners had previously rejected the case, forcing the UDD to file the case with the Department of Special Investigation which later took the case to the Criminal Court.
That court in August 2014 dismissed the case, in which Mr Abhisit and Mr Suthep were charged with murder and attempted murder in connection with the 2010 crackdown on the red-shirt protesters of the UDD. Mr Abhisit was the then prime minister and Mr Suthep his deputy. The Appeal Court later upheld the ruling.
The Supreme Court also upheld the lower courts' ruling to drop the case, reasoning the DSI had no authority to investigate and file charges against the pair saying it was the jurisdiction of the NACC.
Current NACC chairman Pol Gen Watcharapol Prasarnratchakit recently said the NACC would revive the investigation if new evidence came to light.
Mr Nattawut said Mr Abhisit and Mr Suthep can still be charged with malfeasance in office, through Section 157 of the Criminal Code.
Under the law, he said, politicians may be charged if they are found to have injured people by wrongfully exercising their duties.
"If the NACC's processes [concerning this case] are found to be dishonest in any way, we will submit a petition numbering at least 20,000 persons required by law requesting a change of commissioners," he added.
Mr Nattawut said the UDD will also consider distributing information regarding comparisons of the different ways the NACC has treated legal cases involving yellow shirts and red shirts to both domestic and foreign media.
On Wednesday, National Council for Peace and Order (NCPO) spokesman Col Piyapong Klinpan warned the UDD to refrain from stirring up political conflict, as he warned the group against holding Thursday's press briefing in which they set out the case for reviving the probe.
"The NCPO is trying to maintain a peaceful environment to support government initiatives in administrating the country," Col Piyapong said then.
"We ask for [the public] to refrain from any politically-instigative behaviour at this time, and wait for a more fitting time to act."
Mr Nattawut denied the UDD briefing was political or aimed at rallying supporters. The NCPO sent a small number of police to supervise, but no overt attempt was made to stop it.
"Our announcement does not aim to cause any political conflict or challenge any people in power," Mr Nattawut said.