ISLAMABAD, Pakistan — Pakistan’s powerful military has seemed unstoppable recently in getting what it wants: its favored candidate as prime minister, a victory of sorts in a dogfight with the Indian military, and its media critics hounded into silence or compliance.

Now, the Pakistani Army has turned its sights on ending one of the last unbending sources of dissent to its power, an ethnic Pashtun rights protest movement known as P.T.M. that has called the Pakistani security forces to account for extrajudicial killings and other injustices.

Trying to stop a rights demonstration in the troubled region of Waziristan on Sunday, the security forces killed at least eight people, according to members of the movement who described the dead as unarmed and peaceful protesters. The army also took into custody Ali Wazir, one of the movement’s leaders and a member of the country’s Parliament.

After months of threatening and arresting leading P.T.M. figures, the army has made clear that it will no longer tolerate the group. A military spokesman said that its members attacked and wounded security forces before any shooting started — though the army has shown no evidence to counter witness accounts and videos largely pointing to the contrary. The military also accuses the protest movement of being a proxy for enemies in India and Afghanistan.