Among those targeted is Thaksin's chief political adversary, media mogul Sondhi Limthongkul, who last year initiated a campaign accusing Thaksin of corruption and abuse of power.
Thaksin's lawsuit concerns allegations printed in Sondhi's newspaper and on his Web site charging that Thaksin's Thai Rak Thai party met to discuss political change for Thailand, including reducing the influence of the monarchy.
Accusing someone of attacking or insulting the monarchy is political dynamite in Thailand, where 78-year-old King Bhumibol Adulyadej is regarded as the nation's unifier and revered by practically all citizens.
Thaksin also filed criminal defamation charges against executives of Sondhi's Manager Daily newspaper, former senator Chermsak Pinthong and two academics - Pramote Nakornthap and Chai-anan Samutvanij - who repeated the allegations at a public forum, according to the AP.
Criminal defamation carries a maximum penalty of two years in prison and a fine of 200,000 baht (US$5,240, Ђ4,087).
Thaksin has once before filed a defamation suit against Sondhi for accusing the prime minister of usurping royal power, but the case was withdrawn after the king suggested that the country's leader should be more tolerant of criticism.
Sondhi's campaign against Thaksin gained popular support early this year, leading to street protests demanding he resign over alleged corruption and abuse of power.
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