21 persons were injured in a series of grenade blasts on Friday and Saturday in the town of Kayanza in northern Burundi, reports say.
Opposition parties have been blamed for the attacks after they pulled out of the 28 June presidential poll and alleged vote-rigging in earlier local elections.
The worst grenade attacks took place on Friday night in the town of Kayanza in northern Burundi. The first three grenades thrown in bars left 17 injured. Almost simultaneously, another was blasted near a butcher's shop and injured three.
Police sources say five other grenades exploded in Kayanza on Saturday night, injuring one person.
This time, the attacks targeted the homes of local opposition leaders in Kayanza, namely the main parliamentary opposition Frodebu and National Liberation Forces (FNL). Four suspects are already in police custody for questioning.
A spate of grenade attacks has rocked the country, since controversial local elections were held on May 24.
Speaking on local radio stations on Friday, an army spokesman termed them "acts of sabotage."
On Saturday, Burundi head of state Pierre Nkurunziza warned "anyone in possession of firearms to surrender them to the police before June 25, or face sanctions."