Female students at the Jamia Hafsa madrassa religious school say they kidnapped the group as part of an anti-vice campaign.
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Tensions escalated Wednesday when police arrested teachers linked to the kidnappers, and then male students responded by kidnapping two policemen.
The two sides later reached agreement, and the detained teachers and police were freed. But madrassa clerics say there will be no such deal for the hostages from the alleged brothel.
Teachers at the religious school, which is frequently critical of Pakistan's government, say they will not free the group unless police agree to charge them as criminals.
The standoff with police is the school's latest anti-vice campaign in Islamabad, which is more secular than religiously conservative regions near Afghanistan.
Students from its madrassa have held a sit-in at a nearby children's library since January to protest the government's decision to demolish illegally constructed mosques with ties to Lal Masjid - Islamabad's most conservative mosque.
Some information for this report was provided by AFP and Reuters.