Police said the gunmen opened fire on the people coming out of the mosque in the city's Eastern Bypass area after offering Eid prayers.
Former Provincial Minister Ali Madad Jatak, a Pakistan Peoples Party leader, was the target of the assailants but he escaped unhurt, police said.
The incident occurred a day after a suicide bomber killed at least 38 people, mostly policemen, at the funeral of a slain police personnel. Nearly 50 others people were injured in yesterday's attack.
Prime Minister Nawaz Sharif strongly condemned Quetta firing incident.
Deploring the loss of precious lives in the incident he said the nation is facing the monster of terrorism valiantly and expressed the confidence that the menace of terrorism will be overcome soon.
Quetta, the capital city of the troubled Baluchistan province, has been hit badly by militancy and sectarian violence.