Thaksin has asked his lawyers to file the suit against the Democrats, who rallied on Saturday and accused the former prime minister of having a role in the political violence, Pheu Thai Party spokesman Prompong Nopparit said.
Those targeted are: Democrat leader Abhisit Vejjajiva, deputy leader Korn Chatikavanij, Democrat MP for Surat Thani Suthep Thaugsuban, Songkhla MP Sirichok Sopha and Rayong MP Sathit Pitutecha.
The Democrat Party rallied in Lumpini Park on Saturday to condemn the government and provide its account of the military crackdown against red shirt protesters in 2010 when it was in power.
Party members attempted to shed light on the mysterious men in black who had been linked to the red-shirt United Front for Democracy against Dictatorship (UDD) and who were believed by some to have attacked security forces and killed people during the political violence that ensued.
Mr Prompong said Thaksin had no knowledge of the men in black and the Democrat Party was attempting to smear him.
"If the Democrats continue to organise such rallies, this will only turn the public against Thaksin and Pheu Thai," Mr Prompong said.
Mr Prompong said he and the lawyers representing Thaksin will file the defamation lawsuit with Lumpini police station tomorrow.
Meanwhile, Deputy Agriculture and Cooperatives Minister Nattawut Saikuar yesterday challenged the Democrat Party to provide facts about the men in black.
The ruling Pheu Thai Party and the opposition Democrats should have a public debate about the so-called men in black so the public can decide whose side of the story is accurate, said Mr Nattawut, a co-leader of the UDD.
He said the Democrats' rally on Saturday was an attempt to discredit Pheu Thai, so the Pheu Thai Party should explain the truth.
He said he and Jatuporn Prompan, another red shirt co-leader, will use the pro-Thaksin Asia Update television station to get their message out. Democrat leader Abhisit Vejjajiva and Democrat MP Suthep Thaugsuban can use their Blue Sky Channel to give their version of the military crackdown against red shirt protesters in 2010.
Meanwhile, Mr Prompong and relatives of the victims killed during the 2010 political violence yesterday arrived at the National Anti-Corruption Commission to follow up on the NACC's probe in the dispersal of red-shirt demonstrators two years ago by the Abhisit government. Mr Prompong said there had been no progress in the probe.
On May 25, 2010, the Pheu Thai Party filed a complaint with the NACC accusing then-prime minister Mr Abhisit and his then-deputy Mr Suthep of mishandling the red shirt protests.
Mr Prompong earlier criticised the NACC's decision to press charges against those involved in a crackdown on yellow-shirt protesters in Bangkok four years ago when so little progress had been made in its probe into the Abhisit government's dispersal of red-shirt demonstrators.