Police said there were no details yet about the target.
Civilian and military security forces deployed in Balochistan have done little to investigate attacks on Hazara or take steps to prevent the next attack, says a Human Rights Watch report.
Islamic State group jihadists have claimed responsibility for twin explosions.
The attack is believed to be targeting the Shiite Hazara ethnic minority community.
There were no immediate reports of casualties from the bombing, which was felt across the city.
At least 53 people have been killed and nearly 140 injured in Pakistan in separate attacks, including a deadly suicide bombing in restive Quetta city.