Clashes between Burundian police and an "armed group" in the capital city of Bujumbura have left at least six people dead, including one officer.
Police sources said at least three grenades exploded and gunfire was used in the Cibitoke district, which is the opposition stronghold.
The incident is the latest in weeks of unrest, and comes as the country awaits the results from the June 29 elections that were boycotted by the opposition and widely condemned.
The election commission claimed a huge turnout in the local and general elections, despite many polling stations seeming to be quiet.
Despite international outcry, Monday's elections will be followed by the presidential vote slated for July 17.
This is while UN Secretary General Ban Ki-moon on Friday urged Burundi to postpone the vote after the opposition announced a boycott, but the government rejected his appeal.
The African country began to witness political turmoil months ago, when President Pierre Nkurunziza told the public about his plan to run for a third consecutive five-year term in office.
The Burundian opposition says the president can only serve for two terms and Nkurunziza's attempt to extend his 10-year leadership is unconstitutional.
In 2005, Nkurunziza was elected as president and in 2010, he was re-elected with more than 91 percent of the votes amidst an opposition boycott.
The Republic of Burundi is a small country in Africa’s Great Lakes region and shares borders with the Democratic Republic of the Congo, Rwanda, and Tanzania.
Until now, weeks of protests have claimed the lives of more than 70 people and injured about 500 others.
Tens of thousands of people have also fled Burundi since Nkurunziza's renewed presidential bid in April.
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