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 Tuesday 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.
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”.
Who is Pierre Nkurunziza?
- Born in 1964
- Rebel leader-turned president
- Born-again Christian
- Former sports teacher
- Cycles and plays football
- Married with two children
- Father killed in ethnic violence in 1972
Three minor opposition leaders are running for the presidency. Mr Nkurunziza’s four main rivals, including Agathon Rwasa, are boycotting the poll, but the electoral commission has kept their names on the ballot paper.
The African Union (AU) has not sent observers – the first time it has taken such a stance against a member state.
It says the security climate does not allow for free and fair elections.
The European Union has expressed a similar view, and has cut some aid to Burundi to show its anger with Mr Nkurunziza.
At least 70 people have been killed in protests since he announced in April that he was running for a third term.
In May, army generals opposed to his continuing rule failed to overthrow him in a coup.
Critics say that a win for President Nkurunziza would be a hollow triumph that will result in him governing a bitterly-divided nation.
Tensions between Burundi’s ethnic Hutu majority – comprising some 85% of the 10.5 million population – and the country’s Tutsi minority have flared up regularly since independence from Belgium in 1962.
Mr Nkurunziza led a Hutu rebel group fighting the Tutsi-dominated army until a peace deal led to him becoming president in 2005.
The Constitutional Court has backed his argument that his first term in office did not count towards the two-term limit, as he was elected by MPs.
Some 3.8 million Burundians are eligible to vote.