"A grenade was thrown in the car park of the Healing of the Souls church in Nyakabiga," mayor Freddy Mbonimpa said yesterday, referring to a neighbourhood in the city-centre.
"They were very lightly injured by the explosion and their vehicle was badly damaged. They only had a few scratches and were discharged after treatment in a hospital," he added.
The mayor said "armed criminals" were behind the attack, a term used by Burundian officials to refer to members of rebel groups opposed to President Pierre Nkurunziza.
Numerous politicians, civil society leaders and army officers have been killed or narrowly escaped attacks since the start of the crisis.
Such attacks are never claimed, with both sides systematically denying any responsibility.