Pakistani authorities on Monday arrested a founding member of an ethnic Pashtun movement on terrorism charges and for making anti-government speeches, police sources said.
"He was arrested today and he is likely to appear in court tomorrow (Tuesday)," the source added.
Mobile phone footage shared by the PTM appears to show Mehsud being rounded by up police.
Amnesty International denounced the "disappearance" of Mehsud and called on the Pakistani authorities to bring him to court or release him immediately.
Police on Sunday said they were filing complaints against Mehsud and a dozen other activists from the movement including its leader Manzoor Pashteen for making inflammatory speeches against the state at the demonstration.
The PTM has rattled the military since it burst onto the scene early last year with a call to end alleged abuses by security forces targeting ethnic Pashtuns in the restive northwestern tribal areas along the border with Afghanistan.
The area, once plagued by militancy and unrest, is where Washington believes that Pakistan is providing safe haven to militant groups including the Afghan Taliban and the Haqqani Network.
The army has carried out multiple operations in the region, and security there and across Pakistan has dramatically improved in recent years.
But the military maintains a heavy presence still, and the PTM has unleashed festering anger over the alleged abuses against Pashtuns across the country, including disappearances and targeted killings.
The movement remains peaceful but has been notable for its direct verbal attacks on the armed forces in a country where such criticism is largely seen as a red line.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)