Widely circulated security camera footage appears to show three uniformed soldiers grab Sirawitch Sereethiwat on a busy street outside his university yesterday.
The student, who has been a perennial thorn in the military's side since it seized power in 2014, is seen frog-marched into a white car and driven away in front of stunned bystanders.
He spent the night in military custody and was charged Thursday at a Bangkok police station with violating a junta ban on protests and political gatherings, Pawinee Chumsri, of Thai Lawyers for Human Rights, told AFP.
"He was abducted and physically assaulted," Pawinee said, repeating the allegations.
Also Read
"This is an enforced disappearance and arbitrary detention. It's illegal."
Three other students -- all members of the New Democracy Movement -- were also arrested when they visited the police station and face action in a military court.
The four have been charged with violating the junta's ban on public gatherings by organising a trip to a controversial multi-million-dollar park built by the military that has been dogged by corruption allegations.
Outside of their activities there have been relatively few high-profile anti-coup protests across Thailand with the military tightening its chokehold on the country.
A junta spokesperson confirmed the arrest but denied the allegations of assault.
"He was treated leniently and in accordance with the law, there was no violence," Colonel Winthai Suvaree told reporters, adding that the activists have "refused to cooperate" with authorities.
Winthai accused Siriwitch of trying to "provoke the authorities", adding: "I don't know the real reason for this."
Thaksin and his parties have won every Thai election since 2001, facing two coups and the removal by the royalist courts of three premiers in that time.
His sister, Yingluck, was booted from office by a court ruling days before the May 2014 coup.