Former education minister Chaturon Chaisaeng will be indicted in a military court for defying the National Council for Peace and Order's summons to report to the junta chiefs, NCPO spokesman Winthai Suwari said.
Mr Chaturon was arrested on Tuesday afternoon after a press briefing at the Foreign Correspondents Club of Thailand (FCCT) in Bangkok.
Col Winthai said Mr Chaturon had defied the NCPO's order to report in, and statements he made earlier to the public had stirred up unrest, in violation of the NCPO's instructions.
Mr Chaturon's press briefing at the FCCC was planned to receive wide media coverage, he said.
The NCPO was confident it could convince foreign correspondents that the action taken against Mr Chaturn was lawful and justified.
Mr Chaturon's conduct was a violation of martial law, and he would be indicted in a military court, Col Winthai said.
The NCPO spokesman reaffirmed that former prime minister Yingluck Shinawatra had been released, under the conditions announced by the NCPO. If she wanted to travel abroad she would have to seek permission from the NCPO, he said.