Zedi Feruzi was the head of the Union for Peace and Development (UPD) and was walking home with a bodyguard in Bujumbura's Ngagara district when he was attacked by gun wielding men.
An AFP reporter at the scene saw the bloodied bodies of both Feruzi and a police bodyguard lying outside the house soon after the shooting.
Residents said a third person, believed to be another police officer tasked with protecting Feruzi, was seriously injured in the incident.
"We heard around 20 gunshots, everyone fell to the ground, people saw a Toyota car speeding away," said a neighbour, who did not witness the shooting himself.
Around an hour after the attack police had yet to arrive at the scene. Meanwhile, two barricades of tyres were set alight in the district, where local youths were sealing off streets and alleyways to outsiders.
The killings, a day after a grenade attack on a busy market in the capital killed three people, risk further fuelling tensions in Burundi after weeks of violent protests triggered by the president's controversial bid for a third term.
Source: AFP