Quetta is the provincial capital of the restive province of Balochistan where separatist insurgents and Islamist militants are active.
No group has claimed responsibility for the blast.
"The death toll could increase as some of the victims were seriously injured," Shafqat Janjua, a police official, told Reuters.
Mineral and gas rich Balochistan is at the centre of the $60 billion China Pakistan Economic Corridor, which is itself part of China's Belt and Road infrastructure project.
Violence in Balochistan, which borders Afghanistan and Iran, has fuelled concerns about the security of projects such as a planned energy link from western China to Pakistan's southern port of Gwadar.
Five people, including two policemen, were killed and 27 injured in a blast near a police station in Quetta in July, a week after a similar explosion killed two people.