A bomb went off on a bus in the southern Pakistani city of Karachi on Saturday killing six people and wounding 48, police and a hospital official said.

Pakistan's commercial capital and biggest city has seen numerous militant attacks over the past 10 years and is also plagued by violence between rival ethnic-based factions.

The bus sustained serious damage in the explosion and a subsequent fire. While police said the bomb had been planted on the bus, provincial official Sharfud Din Memon said it was left on a motor-bike and went off as the bus passed.

Eight of the wounded were in critical condition, said Seemi Jamali, a doctor at Jinnah Hospital.