The daughter of former Pheu Thai MP Watana Muangsook has submitted a letter to the European Union office in Bangkok to push for her father's release.
In the letter, Ms Weerada asked the EU to help determine whether Mr Watana had violated human rights.
Mr Watana was taken into military custody on Monday or making allegedly inappropriate political remarks. As of Tuesday morning, his whereabouts were unknown.
The military said in a statement that Mr Watana flouted an NCPO order, and committed an act deemed harmful to social order. The military has said that it feared new political conflicts could erupt if people are allowed to express divisive opinions.
Ms Weerada has since tried to find out where her father is and pushed for his release.
She was accompanied on Tuesday to the Delegation of the European Union to Thailand by Narinpong Jinapak, Mr Watana's lawyer, and Kittiratt Na Ranong, a former deputy PM and commerce minister in the Yingluck Shinawatra government.
"I'll go ask officers at the 11th Military Circle when my father will be released and where he is now so I can bring him medicines and food," said Ms Weerada, 19.
Ms Weerada, who will begin her study at the Fashion Institute of Technology in New York in August, planned to go to the US embassy in Bangkok on Wednesday morning and to other places until her father is released.
"Officers won't tell me where they keep him and I'm worried about his safety," she said.
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Mr Watana has no intention to take refuge abroad, she said, dismissing earlier speculation that Mr Watana's persistent defiance was aimed at paving the way for an asylum abroad and to avoid a lese majeste charge.
Mr Wattana was detained after he criticised the draft constitution on Facebook.
Mr Narinpong, his lawyer, said his expression of opinions did not violate the agreement Mr Watana had made with the junta as a condition for releases in earlier detentions.
Besides, he has never incited people to assemble or create unrest. The detention is therefore against the law and in violation of human rights, said Mr Narinpong.
A National Council for Peace and Order source said on Tuesday afternoon Mr Watana was being detained at the 9th Infantry Division Headquarters (Surasee Base) in Kanchanaburi province.
"He's well taken care of and staying in an air-conditioned room but he won't come out much. We're trying to persuade him to cooperate or he might face a lawsuit of violating NCPO orders.
"We don't know how long he will be kept there as it depends on how soon he cooperates. But we think it shouldn't be more than seven days," the source said.