Abdul Rashid Ghazi's allegations could not be independently confirmed, the Indo Asian News Service said.
Rashid said 310 women and 25 men died inside Lal Masjid, known as the Red Mosque, overnight when Pakistani forces attempted to blow holes in the compound's walls.
The Pakistani army hoped the holes would allow students inside to escape, said a military statement.
A Pakistani colonel died Saturday night in the heaviest firing in the six-day seige, BBC News reported. The colonel was shot by students inside the mosque as his forces advanced on the wall, the BBC said.
In a statement Saturday, Pakistani President Pervez Musharraf said the students had no option but to surrender.
As many as 1,800 students remained in the mosque Saturday, BBC said.