The shooting came a day after a jacket wallah targeted a police funeral and killed 38 people, mostly coppers, in the same city.
"Four gunnies opened fire when people were coming out of the mosque after saying Eid prayers," senior local police official Bashir Ahmad Brohi told AFP.
Brohi said a former Pakistain Peoples Party (PPP) provincial minister, Ali Madad Jattak, was also in the mosque and could have been the target.
"But we are not sure at the moment and are investigating, "he said, adding that Jattak escaped unhurt although bullets hit his car.
Another local police official, Sultan Ahmad, confirmed the incident and casualties outside the mosque, which is also a preaching and research centre.
Nobody has so far grabbed credit for the shooting. Most recent attacks in Quetta, the capital of oil- and gas-rich Balochistan
...the Pak province bordering Kandahar and Uruzgun provinces in Afghanistan and Sistan Baluchistan in Iran. Its native Baloch propulation is being displaced by Pashtuns and Punjabis and they aren't happy about it...
province bordering Afghanistan and Iran, have been linked to a Baloch separatist insurgency or sectarian violence.
On Tuesday Baloch separatists rubbed out 14 people including three security officials, 70 kilometres southeast of Quetta.