Shortly before he was held, Chaturon Chaisaeng said he believed the coup would be a disaster for Thailand.
However, Chaturon Chaisaeng said he had no intention of going underground or mobilizing resistance.
On Monday the coup leaders consolidated their legal hold on the country after receiving the endorsement of the king.
The military seized power in Thailand last week, saying it planned to return stability to the country after months of unrest.
The move followed six months of political deadlock as protesters tried to oust the government of Yingluck Shinawatra. At least 28 people were killed and several hundred injured over the course of the protests.
However, the Thai coup – which removed an elected government – has drawn widespread international criticism.
Chaturon Chaisaeng is one of more than 100 opposition figures, academics and activists summoned to report to the military after the coup.
Many of those who have chosen, unlike Chaturon Chaisaeng, to report voluntarily are still in military custody.
Chaturon Chaisaeng was detained in front of journalists at Bangkok’s Foreign Correspondents’ Club where he had emerged from five days of hiding to give a press briefing.
Former PM Yingluck Shinawatra was among those taken into custody after the coup but a military spokesman told AFP news agency she had now been released.
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