Islamabad, Pakistan – A prominent Pashtun rights activist has been presented in a Pakistani court after being arrested on charges of rioting and inciting hatred at a protest demonstration, rights activists say.

Alamzeb Mehsud, 26, was arrested in Karachi, Pakistan's largest city, on Monday evening, video footage taken by activists from the Pashtun Tahaffuz Movement (PTM) showed.

Mehsud's vehicle was intercepted by police on a busy thoroughfare, with armed police officers forcing him to disembark and be taken into custody, the footage showed. An unidentified man, wearing plain clothes, was seen waving a pistol at Mehsud in the footage.

"He was presented in court today [Tuesday] and the court has ordered he be kept in police custody for four days," said Mohsin Dawar, a PTM leader and member of parliament.

Since early 2018, the PTM has organised dozens of mass protests against rights abuses allegedly committed by the Pakistani military in its war against the Tehreek-e-Taliban Pakistan and its allies.

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The country has been battling the Pakistan Taliban, an umbrella organisation of armed groups targeting the state and aiming to enforce a strict interpretation of Islamic law, since 2007.

A series of military operations since 2014 has seen the group displaced from its erstwhile headquarters in the country's northwest and pushed into neighbouring Afghanistan.

Violence has dropped drastically, although sporadic large casualty attacks targeting civilians and security forces still occur.

The PTM and other rights groups allege the military has carried out a campaign of thousands of enforced disappearances and extrajudicial killings as part of its war against the TTP.

Mehsud, a founding member of the group, has been instrumental in gathering data on missing persons and victims of landmines in the northwest tribal districts, where the military's fight was focussed.

On Tuesday, images from his court appearance showed Mehsud in handcuffs, his face hooded. He has been charged under anti-terrorism laws with inciting a riot, defamation and "promoting enmity between different groups", according to the police report filed on his arrest.

On the weekend, Mehsud had addressed a PTM-organised rally of hundreds of protesters in Karachi, repeating the group's calls for justice to be done for victims of enforced disappearances and extrajudicial killings.

The police report named him and 15 others, including Ali Wazir, a PTM leader who is also a member of parliament, in the case.

Amnesty International, a UK-based rights organisation, said it was "concerned" after news of the arrest broke.

We are concerned about reports of the disappearance of PTM activist Alamzeb Mehsud. His whereabouts must be disclosed immediately. Either produce him in court or release him without delay. — Amnesty International South Asia (@amnestysasia) January 21, 2019

"Freedom of peaceful assembly must be protected. Activists must never be attacked," the group said in a tweet.

The PTM has been subject to widespread repression since it launched its movement last year, with leaders regularly named in treason and rioting cases, and coverage of its rallies all but blacked out on local news media.

Dawar, the member of parliament, said the group was undeterred by the government's actions against them.

"If they think that [police cases] and arrests will stop the PTM, they are mistaken," he told Al Jazeera. "They can put as much pressure as they want, we will stick to our demands."

Asad Hashim is Al Jazeera's digital correspondent in Pakistan. He tweets @AsadHashim.