NDM activist Apisit Sapnapaphan told The Nation that Rangsiman was approached by officers from Chana Songkhram police station at 4.30pm. He was alone at the time, reading books at the Bangkok City Library, Apisit said.

According to the group Thai Lawyers for Human Rights, who assisted Rangsiman with the case, the officers showed Rangsiman an warrant issued last August, which called him to see prosecutors regarding his alleged violation against the junta’s order on political gathering.

Rangsiman’s constant denial to approach prosecutors, as he stood firm that he did not break any rule of law from the start, might cause the warrant to be issued, the lawyers group noted.

But it is still unclear why the last August’s warrant came in use just today, the group added.

The warrant was issued around a month after Rangsiman were released from Bangkok Remand Prison, where they had been detained for 12 days for breaking the junta’s order and the Referendum Bill.

On June 23 last year, Rangsiman was charged with other 12 activists after they distributed leaflets with messages against the charter draft at a market in Samut Prakan.

The act allegedly violated the ban on political gathering of more than four people and the then-enforcing Referendum Bill, which prohibited any activity that would lead to certain choices of vote prior to last August’s referendum on the government’s charter draft.

Six activists were released on bail on the following day while the rest, including Rangsiman, refused to apply for bail, causing them to be detained.

On Sunday, Rangsiman was brought to Chana Songkhram police station, where officers recorded an arrest, before taking him to Bang Sao Thong police station in Samut Prakan, where the warrant was issued in relation to his “offence”.

Officers at Bang Sao Thong police station did not immediately comment on the matter.

The warrant referred to an event over a year ago, but Rangsiman claimed to not have seen it before, Apisit said.

Rangsiman assumed that the arrest could have been organised to stop him from filing a petition at Government House on Monday – asking the government to reveal details of the rail project.

The high-speed rail line has become controversial because the junta had agreed to provide a range of legal exemptions for Chinese “experts” who will be involved in the project.

Another NDM activist, Sirawit Serithiwat, said that the plan to file a petition about the rail project on Monday would proceed without Rangsiman.

Sirawit and the NDM, meanwhile, were attending an event entitled “Start Up People, Start Up Talk” at Khok Wua intersection to commemorate 85th anniversary of the Siamese Revolution that moved the country from an absolute monarchy to a constitutional monarchy.

Rangsiman was also due to speak at the event, but had not been released as of press time after 8pm on Sunday.

The event, aimed at raising fund for NDM’s activities and to help activists who have been arrested, was monitored by around 50 uniformed and plainclothes police.

The NDM was reportedly permitted to stage the event on conditions that they must not make politically provocative speeches, hold any political activity, and lead people in from outside the event compound.