The roof of the factory collapsed following a cylinder explosion, initial reports said.
Rescue and relief teams arrived at the scene and evacuated the people trapped under the debris. The injured were transferred to hospitals for medical attendance.
According to Rescue 1122 sources, the rescue workers transferred 21 injured to Civil Hospital Gujranwala for medical treatment.
The rescuers were still searching for those still trapped under the rubble.
The local administration declared emergency at the city hospitals after the incident.