Burundi is heading to the polls with President Pierre Nkurunziza widely expected to win a third consecutive term despite international condemnation and thousands of people fleeing feared violence.
Opposition and civil society groups have denounced his candidacy as unconstitutional and a violation of the 2006 peace deal that ended a dozen years of civil war and ethnic massacres in 2006.
Hours before the polls were due to open on Tuesday, explosions and gunfire rang out late Monday in the capital Bujumbura, the epicentre of three months of anti-government protests.
AFP journalists heard at least three explosions and sustained bursts of gunfire shortly before 1pm NZT.
Earlier Monday, a grenade was lobbed from a passing car into a street close to the symbolic Independence Square in central Bujumbura. There were no reports of injuries.
Willy Nyamitwe, Nkurunziza's chief communications adviser, condemned the attacks as "terrorist acts" aimed at "intimidating voters".
Critics fear a win by the incumbent will be a hollow victory, leaving him ruling over a deeply divided nation.
"Despite a facade of pluralism, this is an election with only one candidate, where Burundians already know the outcome," said Thierry Vircoulon from the International Crisis Group, a think-tank that has warned the situation has all the ingredients to kick-start renewed civil war.
With the elections denounced by the opposition as a sham, the 51-year-old president - a former rebel, born-again Christian and football fanatic - is facing no serious competition.
More than two months of anti-Nkurunziza protests, which were violently repressed, have left at least 100 dead since late April.
Independent media has been shut down and many opponents have fled - joining an exodus of over 150,000 ordinary Burundians who fear their country may again be engulfed by widespread violence.
Doctors Without Borders (MSF) said Monday about a thousand people were fleeing each day into Tanzania, crossing the border "through the forest... many travelling in the dark on foot and without belongings."
In mid-May, rebel generals attempted to overthrow Nkurunziza in a coup. After that failed they launched a rebellion in the north of the country.
Last-ditch crisis talks mediated by Uganda broke down on Sunday.