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Thailand's military government has arrested alleged sympathizers of a tiny political movement advocating the abolishment of the constitutional monarchy, seizing as evidence T-shirts bearing the group's logo of a red and white flag.

The legal aid group Thai Lawyers for Human Rights said Tuesday that at least five people were detained by the military in the past week, and three charged with sedition.

The group they allegedly belong to, the Organization for Thai Federation, is active mostly online with supporters abroad, and has no public presence in Thailand. It advocates a federal system of administration along with abolishing the monarchy.

Republicanism is a taboo subject in Thailand, where citizens are inculcated with love of the monarchy.

However, the palace has been dragged into partisan political battles over the past decade, jeopardizing its past status as an untouchable subject for criticism or debate. The death in 2016 of King Bhumibol Adulyadej, who reigned for 70 years, has also affected its standing.

Prime Minister Prayuth Chan-ocha told reporters Tuesday that the detained people are part of an anti-establishment group that has been vocal on social media despite the government's efforts to shut them down. He said their arrests were necessary.

"Thailand is not a federation. We are a democratic constitutional monarchy and it is stipulated in the constitution that we are one indivisible kingdom," Prayuth said. "But a federation divides into many states, which Thailand does not. And the Thai flag is the tricolor flag. So can their actions be considered treacherous?" he asked reporters.

The dissidents' flag logo appears to be inspired by the Thai national flag, which has horizontal bars of red, white and blue. The group's logo, with vertical stripes, does not include blue, the color understood to represent the monarchy.

At least some of the members of the Organization for Thai Federation appear to have belonged to an extreme faction of the Red Shirt movement, supporters of former Prime Minister Thaksin Shinawatra, who was accused of disrespect for the monarchy and ousted by an army coup in 2006. In the aftermath of that takeover, the Red Shirts fought sometimes violent street battles against Thaksin's opponents, who were backed by the military.

Deputy Prime Minister Prawit Wongsuwan also commented Tuesday on the T-shirt arrests, saying that those detained were part of a network of outlaws that had fled to Laos to evade charges of sedition and lese majeste, insulting the monarchy.

"They come from one source, which is from the Laos side, and they also have a network in Thailand," Prawit said.

"They sell shirts and flags. We've arrested three or four of them now. Whoever we can get to, we will arrest them all," he said, adding "They are traitors."

Thaksin's sister, Yingluck Shinawatra, became prime minister in 2011, but the army staged a coup against her government in 2014, and has since kept a tight lid on dissent against its rule, including peaceful demonstrations for democracy.

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This story has been corrected to show that only three people have been charged so far.