The roof collapse happened in Daroghawala, a low-income neighbourhood in the outer city with narrow and congested streets.
"The bodies of at least nine worshippers have been pulled out from the rubble of the roof, while an unknown number of people are still buried," rescue official Jam Sajjad told the AFP news agency.
There were up to 30 people at the daily afternoon prayer.
"We fear that the death toll might rise. The roads there are very crowded, preventing heavy machines from entering the area to remove the debris. The people of the area are also helping the rescue effort," Sajjad said.