About half a dozen soldiers took education minister Chaturon Chaisang into custody in a chaotic scene at the Foreign Correspondents' Club of Thailand, where he had just finished giving a surprise news conference.
The junta, which seized power on Thursday, is holding most senior members of the elected administration and has ordered the rest to surrender.
Mr Chaturon called for elections and warned that resistance to the army overthrow could grow, which could lead to "a disaster for this country".
When the news conference was finished and Mr Chaturon was being interviewed by a group of Thai journalists, soldiers entered the room, surrounded him and escorted him out through a crowd of reporters. He was calm and smiling as he was taken away.
Before being hustled into a lift, Mr Chaturon said: "I'm not afraid. If I was afraid, I wouldn't be here."
The military takeover, Thailand's second in eight years, deposed an elected government that had insisted for months that the nation's fragile democracy was under attack from protesters, the courts, and finally the army.
The country is deeply split between an elite establishment based in Bangkok and the south that cannot win elections, and a poorer majority centred in the north that has begun to realise political and economic power.