No group has claimed responsibility for the explosion which comes a week after the country's Labour and Social Services minister Duran Farah Ahmed was targeted by a blast from a landmine.
His convoy was using one of Mogadishu's main junction when the attack took place.
The militant group al-Shabaab has been behind a series of attack on the capital since Somali troops backed by an African Union intervention force seized control of some of their stronghold around the country.
The group has been waging an insurgency for over eight years to overthrow the central goverment and replace it with a puritanical administration based on Islamic law.
Somalia has been rocked by instability since the overthrow of late strongman Siad Barrie in 1991.