Lahore-based academic Dr Ammar Ali Jan was arrested from his residence in an early morning raid on Saturday and was taken to the Gulberg police station.
In a Facebook status posted in the early hours of Saturday, Jan wrote he is "under arrest at the Gulberg police station".
"There is an FIR against me for participating at the protest in Liberty against the killing of Professor Arman Loni," Jan's Facebook post said, adding that he was taken into custody at 4am which is when he said "the police raided my house"
In the FIR registered against Dr Jan, the complainant said that Jan was leading a group of 100-150 people from "Pashtun Ittehad Movement" who had gathered on the call of Manzoor Pashteen.
It added that the gathering was blocking roads and Dr Jan along with some others were chanting slogans against state institutions and intelligence agencies.
During the week, protests have taken place in Quetta, Islamabad and Lahore over the death of senior Pashtun Tahaffuz Movement (PTM) leader Arman Loni who was allegedly killed during a sit-in in Balochistan's Loralai.
On Tuesday, at least 18 activists of the PTM were detained from outside the National Press Club in Islamabad.
Among them was rights activist Gulalai Ismail, who was in detention for nearly 30 hours, and was released allegedly as a result of intervention from senior members of the Pakistan Tehreek-i-Insaf (PTI) government at the centre.
The PTM is a rights-based alliance that, besides calling for the de-mining of the former tribal areas and greater freedom of movement in the latter, has insisted on an end to the practices of extrajudicial killings, enforced disappearances and unlawful detentions, and for their practitioners to be held to account within a truth and reconciliation framework.
Earlier in Nov 2018, Jan, who was scheduled to speak at the Faiz International Festival, was barred from speaking at the event. At the time, he stated that he was informed by the organisers only three hours prior to his sessions that he was no longer permitted to speak due to "unavoidable" reasons.
In April 2018, Jan was also removed from the visiting faculty at the Punjab University, for what the administration says, "failing to meet contract requirements".
It was however alleged that Jan was sacked over his political views and activism.