Police said the apparent target of the attack in Pushtunabad neighbourhood were Afghan refugees who live in the area in large numbers.
"The blast took place at an ice-cream shop at Meetha Chowk in Pushtunabad," Quetta police chief Abdul Razzaq Cheema said.
Rasheed Jamali, a doctor on duty at the Civil Hospital where the injured were taken, said the condition of five persons was critical and the death toll could rise.
Police said the blast in a crowded area damaged nearby shops and parked vehicles. The locally made bomb with an estimated two kilograms of explosives was detonated by remote control, Cheema said.