Black T-shirts with a "separatist movement" symbol seized from a vendor came from a single source in Laos, a group of Thai anti-monarchists fleeing prosecution for lese majeste, according to Deputy Prime Minister Prawit Wongsuwon.
Gen Prawit, also the defence minister, said on Tuesday the group wanted "an administration without the institution".
He was responding to questions about the arrest of two women - one who allegedly sold T-shirts with the logo of the "Thai Federation" on the left chest, and another who wore one of the shirts.
"They came from a single source in the Lao PDR and it has a network in Thailand. There have been communications about selling shirts and flags," Gen Prawit said.
He identified a person behind the movement as Chucheep Cheewasut, who had fled the country. The man was prosecuted for lese majeste and sedition, and security authorities had monitored his activities for a long time, Gen Prawit said.
Three or four people had been arrested, and anyone else who became involved with the movement would also be arrested. "They are rebels," Gen Prawit said.
However, the movement had no significant momentum. It existed only on social media and aimed to inspire people to think about a new state without the institution, he said.
He would ask the Lao government to prevent any support for the movement.
A source at the National Council for Peace and Order said that a woman had brought the T-shirts from Laos, and about 450 were seized from her house.
The small rectangular symbol on the left breast of the T-shirts consists of a broad red vertical bar, with a broad white vertical bar on each side - like a flag.