Attackers shot at a lawmaker from Burundi's ruling party as he drove to parliament on Friday, killing one police officer and wounding others, the lawmaker and a witness said.
One of the attackers who was arrested led police to a large illegal arms cache of rifles, grenades and other weapons.
The attack is the latest in a spate of shootings and attempted assassinations targetting pro-government officials and opposition politicians that have worried Western powers and regional nations, who fear a slide back into ethnic conflict.
Zénon Ndaruvukanye, a former adviser to President Pierre Nkurunziza, escaped but told Reuters that a police officer was killed in the attack and others were wounded.
A gunman stepped out of a vehicle as Ndaruvukanye was driving and fired with an automatic rifle, the lawmaker with Nkurunziza's ruling CNDD-FDD party said. He said a police officer was killed
Another witness gave a similar account, adding that other police officers were also wounded.
Burundi, which emerged from an ethnically charged civil war in 2005, was plunged into a deep crisis when Nkurunziz said in April he would run for a third term in office and then won a disputed July election.
Opponents say another five-year term in office violates the terms of a peace deal that ended the civil war. Burundi has witnessed weeks of violent protests, a failed coup attempt in May and a spate of political killings.
Till now, opposing camps represent rival political allegiances, but experts say the longer the crisis continues the more chance old ethnic divisions will emerge.
The 12-year civil war pitted rebel groups of the Hutu majority against the army which was at the time led by the Tutsi minority. Nkurunziza led one of the main Hutu rebel groups, and Ndaruvukanye was one of his commanders.