ISLAMABAD, Pakistan — The Pakistani authorities on Monday arrested the leader of a civil rights movement who for nearly two years has challenged the country’s powerful security forces by demanding they be held accountable for extrajudicial killings and kidnappings, his colleagues said.

The leader, Manzoor Ahmad Pashteen, was arrested in the early hours of Monday morning on five separate charges, including conspiracy and sedition, after giving a speech that officials said was against the Constitution and the state. He was detained in Peshawar, the capital of the northwestern province of Khyber-Pakhtunkhwa, his colleagues said.

Mr. Pashteen and his movement, widely known by the initials P.T.M., have presented one of the most influential challenges to the military’s dominance of Pakistan as it has cracked down on minorities, journalists and other critics in recent years.

While the P.T.M. focused on demanding justice for the country’s sizable Pashtun minority, its influence quickly grew larger than the movement itself. The large crowds P.T.M. drew to the streets, and the boldness of its leadership in openly challenging the security forces, inspired other advocates to join in.