Authorities are investigating a case involving the distribution of T-shirts bearing the symbol of a "separatist movement" after a woman was recently detained for selling them, Deputy Prime Minister Prawit Wongsuwon says.

He was referring to the arrest by soldiers of a woman identified only as Wannapa at a house in Samut Prakan last Thursday. A number of black T-shirts with a badge looking like a flag featuring vertical bands of white and red on the left side were subsequently confiscated.

The woman was taken to the 11th Military Circle in Bangkok's Dusit district where she has been detained for questioning.

Under NCPO Order No.3/2015, security officers are granted the authority to detain people for questioning for up to seven days. That deadline is set to expire Wednesday in Ms Wannapa's case.

Gen Prawit, who also serves as the defence minister, said Monday the T-shirts belong to a political party and the badge they bear is the symbol of a separatist movement seeking to establish a so-called "Thai Federation".

"Wasn't it necessary for the authorities to make the arrest? Officials are investigating the case now at the 11th Military Circle," Gen Prawit said.

On Thursday the Thai Lawyers for Human Rights Centre reported that a woman identified as Surangkhanang was taken by soldiers from her house in Bangkok's Prawet district in a passenger van at around 7am to the 11th Military Circle, according to Prachatai.com.

The soldiers also searched her house and seized mobile phones from her and her daughter.

The woman was later released. She told the centre that soldiers had questioned her over the T-shirts and told her not to wear them or purchase any more.

She was also asked to sign a document promising she would not take part in any political activities, and confirming that she had been detained without intimidation.

The soldiers told the woman they had also detained the T-shirt vendor.

Surangkhanang's niece told the centre that her aunt had purchased a black T-shirt bearing the controversial logo online several months earlier. Last month, two people looking like soldiers asked to speak with her aunt at her beauty parlour, but the older lady refused as she had clients to attend to.

That same day, Sept 6, soldiers searched Ms Wannapa's home and took her, some T-shirts and a laptop from the house in Samut Prakan at around 7.30am, the centre reported.

Ms Wannapa later told the rights centre she had been brought in for "attitude adjustment" and was unsure when she would be released.

The 30-year-old hails from the northeastern province of Maha Sarakham, works as a motorcycle taxi driver and has two sons aged 9 and 14, according to her husband. The family stays in a rented room in Samrong, Samut Prakan.

Her husband said Ms Wannapa took some of the T-shirts back to their room in August with the intention of selling them to supplement their income.

But he said she was not aware of the significance of the logo and had never attended any political gatherings.

On Saturday, the Organisation for Thai Federation issued a statement condemning the authorities for arresting Ms Wannapa. It said the arrest was meant as an intimidation tactic.

The following day, National Human Rights Commissioner Angkhana Neelapaijit accompanied Ms Wannapa's two sons to visit their mother at the 11th Military Circle.

They were met by Col Burin Thongpraphai, a legal official of the National Council for Peace and Order. No reporters were allowed inside.

After the visit, Ms Angkhana told reporters that soldiers would hand Ms Wannapa over to the police for further legal action.

She said she was informed that Ms Wannapa had been arrested on suspicion of being involved in a separatist movement and the NCPO had evidence to support the charges against her.

She was not shown the evidence, however, and said she urged the soldiers to respect Ms Wannapa's basic rights as no charges had yet been filed against her.