A teen anti-coup activist was held by the military for several hours Thursday, in relation to his attempted visit along with other activists to Rajabhakti Park in Hua Hin on Monday.
According to human rights lawyer Arnon Numpa, police arrived at Chatmongkon Wanli's house at 9.30am, before arresting and escorting the 19-year-old to Bang Mod police station.
He was then transferred to military custody in a nearby facility.
The activist was forced to sign an agreement saying he would steer clear of political activities.
The MoU also requires him to seek permission from the National Council for Peace and Order before travelling abroad.
Mr Arnon said he did not know the reason for the arrest but claimed the young activist had been "invited for lunch", before being driven home about 2.30pm.
Piyarat Chongthep, another activist, claimed the military had been monitoring Mr Chatmongkon since early this month.
Mr Chatmongkon, who has taken part in anti-coup protests since 2014, was called in by the military on Dec 5, about the students' plans to visit Rajabhakti Park, said Mr Piyarat.
Mr Chatmongkon had been warned not to take part in the trip and was required to report to the military on a daily basis but failed to comply, said Mr Piyarat.
On Monday, student activists attempted to stage a symbolic probe at Rajabhakti Park, and read a statement discussing alleged graft in the park project.
However, they were stopped on their way there, after police and soldiers intercepted the group at a railway station in Ratchaburi province and disconnected their carriage from the rest of the train bound for Hua Hin.
Thirty-six members of the group were arrested but Mr Chatmonkon and a few others managed to escape, Mr Piyarat added.
Mr Piyarat said Mr Chatmonkon returned home only Thursday as he had felt unsafe. However, he was promptly arrested.
Since the May 2014 coup, the army has conducted several similar arrests, Mr Piyarat said, where they detain activists first and explain their reasons later.
Only a lawless country would permit such arbitrary detention, he added.
"Mr Chatmongkon's case is not the first, and certainly won't be the last," he said.