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Polling stations have closed and votes have started to be counted in Burundi's controversial presidential elections, in which incumbent Pierre Nkurunziza is widely expected to win a third consecutive term.

Ballots will continue to be tallied on Wednesday, but officials have said that they do not expect the results to be announced until Thursday.

Shortly before voting started on Tuesday, a policeman and a civilian were killed amid a string of explosions and gunfire in the capital Bujumbura, the epicentre of three months of anti-government protests.

Al Jazeera's Haru Mutasa, reporting from Bujumbura, said that one member of the opposition was also killed overnight in the city's Nyakabiga neighbourhood. The incident prompted a big crowd to gather there in protest in the morning.

The body was removed on Tuesday, several hours after the incident. But the crowd who were boycotting the elections still stayed on at the incident spot, with riot police mobilised near them keeping a safe distance.

About 3.8 million Burundians were eligible to vote in the polls, which the opposition and civil society groups are boycotting, claiming they will not be free and fair.

Electoral Commission president Pierre-Claver Ndayicariye said turnout was low in Bujumbura and southwestern Bururi province, but gave an overall figure of 74 percent, comparable to that of last month's general elections.

The opposition have denounced the candidacy of the incumbent president as unconstitutional and a violation of the 2006 peace deal that ended a dozen years of civil war and ethnic massacres in 2006.

A spokesman for the US said the elections lacked credibility and by pressing ahead, the government risked its "legitimacy".

State department spokesman John Kirby said, the vote "will further discredit the government".

The nation's constitutional court has ruled in the president's favour, however, maintaining he is eligible for a third term because he was chosen by legislators - and not popularly elected - for his first term.

Nkurunziza told journalists he was allowing "the Burundian people to vote or to choose someone they believe in". The president arrived to vote in his home village of Buye on bike.

Ban Ki-moon, the UN secretary-general, has called on authorities to do all in their power to ensure security and a peaceful atmosphere during the election.

"He [Ban] further calls on all parties to refrain from any acts of violence that could compromise the stability of Burundi and the region," his spokesman said in a statement on his behalf.

More than two months of anti-Nkurunziza protests, which have often been 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 more than 150,000 Burundians who fear their country may again be engulfed by widespread violence.

Doctors Without Borders (MSF) said on Monday about a thousand people were fleeing each day into Tanzania.

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.

"The government has opted to isolate itself and go ahead with pseudo-elections," said Leonce Ngendakumana, a prominent opposition figure, after talks collapsed.

"They have refused to save Burundi from sliding into an abyss," said Jean Minani, another opposition figure.

Analysts say renewed conflict in the country could reignite ethnic Hutu-Tutsi violence and bring another humanitarian disaster on the region.

The conflict also risks drawing in neighbouring states - much like in the war-torn east of the Democratic Republic of Congo.

The last civil war in Burundi left at least 300,000 dead.