"Police stormed into Jamia Faridia and arrested dozens of students and shifted them to an unknown place," a senior security official told AFP on Saturday morning.
"Whatever happens now, the government will be responsible."
The delegation had received permission from Abdul Rashid Ghazi, the senior cleric at the mosque, to enter the compound and was hoping to persuade him to send out children holed up inside.Martyrdom
Ghazi has declared he would choose "martyrdom" rather than agree to unconditional surrender, and has rejected government accusations that he is holding women and children as human shields.
Officials say they don't know how many students are left inside but they estimate that there are up to 60 "hardcore" armed men. Ghazi has said 1,900 people remain in the compound.
Police have also seized control of the nearby Jamia Faridia, an Islamic seminary affiliated with the Red Mosque.
Authorities said they feared that students from the seminary, three kilometres from the mosque, would open another front as the confrontation entered its fifth day.
At least 19 people have been killed in fighting that began on Tuesday but there have been unconfirmed reports of the Red Mosque's defenders burying more bodies on Saturday.
Dozens of trucks carrying troops moved towards the mosque just before daybreak as gunfire continued amid heavy monsoon rain.
"We are not making any advances, this is part of the operation to secure the release of women and children held hostage by the cleric," a security official said.
Students from the mosque threw grenades and petrol bombs at security forces, who responded by firing, he said.
The government had earlier rejected a conditional surrender offer by Ghazi, whose brother Abdul Aziz, the founder of Faridia seminary, was captured on Wednesday while trying to flee the mosque dressed in a burqa.
"We have decided that we can be martyred but we will not surrender. We are ready for our heads to be cut off but we will not bow to them," Ghazi told the private Geo television station.
Source: Al Jazeera and agencies