People in Burundi are voting amid widespread tension over President Pierre Nkurunziza’s decision to stand for a third consecutive term.
Polls opened at 06:00 local time (04:00 GMT), after a night of gunfire and explosions claimed two lives in the capital Bujumbura.
The president’s critics say the vote is unconstitutional, as he is only entitled to stand for two terms only.
The government accuses the opposition of provoking violent protests.
The BBC’s Maud Jullien in Bujumbura says shooting was continuing in the streets on Monday morning.
Bursts of gunfire could be heard in at least one district of the capital, as polling stations were about to open, she added.
A policeman and a civilian are believed to have died.
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The president’s office has described the latest protests as terrorist acts intended to disrupt the election.
Chief communications adviser Willy Nyamitwe said: “People do it to intimidate voters. They don’t want the voters to go to the polls.”
About 1,000 people are fleeing into Tanzania each day to escape the violence, according to medical charity Medecins Sans Frontieres (MSF).
It said many families were crossing “on foot without any belongings”.