According to BBC, just under three-quarters of the country's 3.8m eligible voters turned out on Tuesday after a night of gunfire and explosions that claimed two lives.
The US State Department has joined critics saying the election lacks credibility.
The government accuses the opposition of provoking violent protests. BBC reports.
President Nkurunziza is running for a third term despite a limit of two terms in the constitution.
Two policemen were shot dead in the capital Bujumbura on Monday night, said Willy Nyamitwe, the president's chief communications adviser.
The body of an opposition official was found earlier on a road.
The president's office described the latest protests as terrorist acts intended to disrupt the election.
Meanwhile in opposition areas of the capital, among the few who voted, many tried to wipe off the indelible ink on their fingers, fearing reprisals from opposition supporters.