At least five people were killed and 40 others wounded in a blast near Cantonment Railway Station, commonly known as Cantt Station in Karachi on Saturday.
The blast was so powerful that the roof of the bus was swept away while window glasses of nearby buildings were broken.
Extensive fire caught the two other buses, motorcycles and nearby shops after the blast. The bodies and injured were shifted to Jinnah Hospital in the city.
According to police, several people were on board before the blast hit the bus which was on its way to Sargodha.
Police officials have cordoned off the area and started investigations regarding the incident, however nature of blast was unknown so far.
Fire brigade vehicles reached late on the spot, which caused more damages.
The Prime Minister and Army chief were present in the city when the blast occurred.
A day before, the government had suspended mobile phone services in the wake of terrorism threat.