The detentions were made Saturday, and the students, aged between 14 to 18 years -- who had no documents giving them permission to be in the country-- will eventually be deported to Afghanistan, senior police official Nadeem Hussain told AFP.
"We have sealed the madrassa as it did not have any registration documents," he said.
Akbar Harifal, the Home Secretary for Balochistan province, of which Quetta is the capital, confirmed the raid and detentions.
With little oversight of what was being taught to the hundreds of thousands of children enrolled in the country's madrassas, fears intensified after the 2014 attack that some religious schools were breeding grounds for intolerance -- or even extremism.
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There was no suggestion that the seminary raided Saturday had links to extremism.
Pakistan is home to 1.5 million registered and about as many undocumented Afghan refugees, with growing insecurity in Afghanistan impeding voluntary return programmes.
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