Gulalai Ismail was detained as soon as she arrived at an Islamabad airport from London.
London-based rights group Amnesty International called for her immediate release saying she was being "detained solely for her peaceful human rights work."
Ismail is a critic of the Pakistani military's operations in the country's tribal regions during which many innocent civilians have been killed.
She has been a supporter of the Pashtun Protection Movement, a group which denounces perceived high-handedness by security forces and their operations in the country's northwestern regions.
It calls for judicial probes into those killed by the military in the name of its war on terror.
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