In the first real hint of the shape the politically fraught country's next administration may take, army sources told AFP that the military will select the 200 assembly members and that the junta itself will not be dissolved.
"We have learned our lesson. By pushing power in other people's hands, they may not do what we expect them to do," said an official under the condition of anonymity.
Earlier this month junta chief General Prayut Chan-O-Cha said the regime would set up an interim government by September to oversee political reforms, including crafting a new constitution, followed by elections in about a year's time.
Pro-coup demonstrators have called for reforms that would rid the country of the influence of the Shinawatra family, whose political parties continually win during elections but are loathed by much of the country's powerful elite.
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The junta - formally known as the National Council for Peace and Order (NCPO) - has now finished drafting an interim constitution, according to the military official.
"The interim constitution is complete... The NCPO's powers will remain - different from the 2006 coup when the military lost power after establishing an interim government," the source said.
The officials did not specify the exact balance of power between the junta and the planned interim government.
The remarks came as a Thai opposition alliance set up to counter the nation's coup-making junta said today it would establish an official base in a Western country by next month.
Thailand's junta has muzzled dissent, summoning and detaining hundreds of people, the majority linked with the deposed government of ex-premier Yingluck Shinawatra and her administration's "Red Shirt" supporters.
Prayut, who is due to retire as army chief in September, has not ruled out becoming prime minister himself.
He also has not revealed whether the cabinet, handpicked by the premier, would be made up of civilians or military personnel.