Fugitive red-shirt co-leader Jakrapob Penkair says an anti-coup organisation-in-exile has been formed.
Mr Jakrapob wrote on his Facebook page yesterday that he and other people against the coup have formed an overseas organisation to fight against dictatorship.
His group would announce in the next few days the "freedom and liberty of Thai people" from the military dictatorship.
He claimed the anti-coup body has been warmly received by the international community, saying not only Thai nationals but also world figures have supported his move to fight the junta.
Mr Jakrapob did not give any details about which foreign country he has chosen to be the base for the organisation-in-exile.
The core leader of the red-shirt United Front for Democracy against Dictatorship (UDD) was among more than 20 lese majeste suspects recently ordered by the National Council for Peace and Order (NCPO) to report in or face trial in a military court.
Mr Jakrapob, a former Prime Minister's Office minister in the Thaksin Shinawatra government who is now living in exile, lashed out at the junta for using unlawful powers to undermine political activism.
He said it was evident Thailand would not return to the democratic path for a long time to come as the military would use the power that it had unlawfully seized to reap benefits for themselves, their families and their cronies.
He said the NCPO's policy to "bring back happiness to the country" was a fraud. He called on Thais to join him in his fight against the dictatorship.
Foreign Ministry spokesman Sek Wannamethee yesterday said the ministry has sought information from the Cambodian government about Mr Jakrapob's movements but Cambodia said it would not interfere in other countries' affairs.
Cambodian Prime Minister Hun Sen had earlier said he would not allow the country to be used as a base for political activities, Mr Sek said.
He admitted the ministry did not know where Mr Jakrapob was staying.