The man leading protests accusing Pakistan’s army of human rights abuses has been arrested for alleged criminal conspiracy and sedition.
Manzoor Pashteen was taken into custody along with nine others from his Pashtun Protection Movement in Peshawar.
Mr Pashteen has drawn tens of thousands to rallies in cities around Pakistan.
A fellow protest leader said he was being punished for simply demanding human rights. The powerful military, unused to criticism, denies wrongdoing.
Mr Pashteen, a charismatic former veterinary student who shot to prominence two years ago, has become the face of the Pashtun Tahaffuz (Protection) Movement (PTM), in a country where few openly challenge the military.
A number of cases highlighted by the PTM and investigated independently by the BBC came to light in a report last year.
“It has taken us almost 15 years of suffering and humiliation to gather courage to speak up, and to spread awareness about how the military trampled our constitutional rights through both direct action and a policy of support for the militants,” Manzoor Pashteen told the BBC.
The non-violent protests began over the alleged extra-judicial killing of a young man of ethnic Pashtun heritage by police in Karachi.
The movement then expanded, demanding accountability from Pakistan’s army for alleged human rights abuses against Pashtuns committed during the war against Islamist extremists in the country’s north-west.
Pashtuns make up the majority of the population along the border with Afghanistan.
The protests, drawing at times tens of thousands of demonstrators, have rattled the military.
Manzoor Pashteen has been accused of “hate speech” and sedition among other offences.
He was taken into police custody in Peshawar but is expected to appear before a magistrate in Dera Ismail Khan, some 300km (186 miles) to the south, where charges against him have been filed.
His fellow protesters have demanded his immediate release. Another PTM leader, MP Mohsin Dawar, urged supporters to remain calm in response to the arrest.
This is the first time Manzoor Pashteen has been held – why the authorities chose to detain him now is unclear.
The authorities have repeatedly arrested PTM leaders and activists since the movement came to prominence. Last year Mohsin Dawar and his fellow PTM member National Assembly, Ali Wazir, were detained after a deadly clash in Waziristan.
A media blackout has ensured the PTM’s peaceful rallies stay off front pages and TV bulletins – although the movement has been successful in getting its message over through social media.
According to authorities and independent research groups, militant violence since 2002 has forced more than five million people in Pakistan’s north-west to leave their homes to seek refuge either in government-run refugee camps or rented houses in peaceful areas.
There are no official figures of the total death toll of this war but estimates from academics, local authorities and activists put the number of civilians, militants and security forces killed at well over 50,000.
Many see the PTM as breaking new ground in the political landscape of a country where proxy wars have disenfranchised large populations not only in tribal areas and the north-west, but also in Balochistan and other parts of the country.