The incident occurred as hundreds of faithful observed afternoon prayers on Friday at the city’s central mosque, Zaria Emirate Council spokesman Abdullahi Kwarbai said, Al Jazeera reported.
“Four bodies were found initially, and then three others were found after the rescue team searched the collapsed mosque,” he said.
State officials said the mosque was constructed in the 1830s.
Twenty-three people were sent to hospital, the state Emergency Management Agency said.
Videos apparently recorded at the scene showed a wide opening where a part of the roof fell in.
Kaduna state Governor Uba Sani ordered an immediate investigation into the disaster and promised to assist those affected by the “heartbreaking incident”.
The incident follows more than a dozen building collapses in the West African nation in the last year alone.
Authorities often blame such disasters on a failure by officials to enforce building safety regulations and on poor maintenance and substandard construction materials.