Senior police official Munir Sheikh said the explosives were planted on a motorcycle and exploded near Murghi Khana bus stop in Quaidabad area of Karachi as the bus of Sindh Police's Special Security Unit was passing by.
"Two policemen were killed and about 13 injured in the attack," he said.
The injured were shifted to the government-run Jinnah Postgraduate Medical Centre.
Dr Seemi Jamali, spokesperson of the hospital, said that 12 wounded persons had been brought into the hospital for treatment and at least one injured was critical.
The government launched an operation in 2013 to eliminate them but they are still active in certain areas.
At least two soldiers of paramilitary Rangers were killed in a suicide attack their vehicle in the city on March 20.