According to Iran's Seismological Centre, the city was later hit by 13 aftershocks.
The temblor, having its epicentre at a depth of 12 km, struck Kaki city of the province in the afternoon, Xinhua reported.
Bushehr province's governor Fereidoon Hassanvand said some 700 buildings were badly damaged.
Twenty rescue teams and several helicopters had been dispatched to the quake-hit region from neighbouring cities and provinces, Mahmoud Mozaffar, chief of Iran's Red Crescent Society said earlier.
In two villages, the damaged area exceeded 80 percent, reported semi-official Mehr news agency citing Mozaffar.