The Pheu Thai Party plans to lodge complaints with the Crime Suppression Division and the Constitutional Court this week in connection with anti-government leader Suthep Thaugsuban's vow to seize power.
Party spokesman Prompong Nopparit said Mr Suthep's announcement that if his anti-government movement gained sovereign status he would seek royal endorsement for a new prime minister, reflects his intention to break laws and abuse the royal decree.
Mr Prompong was referring to the announcement made by the People's Democratic Reform Council (PDRC) leader on Saturday night at Lumpini rally site. He said he intended to seize power as soon as the National Anti-Corruption Commission or Constitutional Court rules against caretaker Prime Minister Yingluck Shinawatra.
Mr Prompong said Mr Suthep has ignored the importance of democracy and the people.
The spokesman said he learned that Mr Suthep had drawn up plans for a power grab with a key figure at a hotel in the Sukhumvit area. They also laid plans to prepare three men as potential candidates to be new prime minister, he said.
Pheu Thai's legal team will file evidence and submit it to the Crime Suppression Division later this week, Mr Prompong said.
The team will petition the charter court, asking it to rule whether Mr Suthep's act is tantamount to an attempt to overthrow the democratic system, he said.
Even if a seizure of power by the PDRC is already under way, Thais will not tolerate Mr Suthep's attempts to destroy the democratic system, he said.
Bhumjaithai Party leader Anuthin Charnvirakul also expressed disagreement with Mr Suthep's announcement.
He said Mr Suthep's plan could eventually damage the democratic system.
If one group voices their attempt to gain sovereign status, others can do so as well, and then there will be no end to the problem, he said.
The party believes the people should decide on the country's future, he said.
Mr Anuthin said he was not worried about the prospect of a coup because all sides understand the need for democracy to be upheld.