Polling stations have closed after Burundi's controversial presidential election on Tuesday, which was marred by a widespread boycotts and the killing of at least two people — a policeman and a civilian, according to witnesses — in a string of explosions and gunfire in the capital, Bujumbura.

An opposition member was also killed overnight in the Nyakabiga neighborhood of Bujumbura, the center of three months of anti-government protests.

Their body was removed early Tuesday, several hours after the incident, which prompted a crowd to gather in protest before voting began. The protesters were boycotting the election and stayed at the scene, with riot police mobilized near them keeping a safe distance.

About 3.8 million Burundians are eligible to vote in the polls, but the country's opposition and many civil society groups are boycotting election, claiming they will not be free and fair. President Pierre Nkurunziza is widely expected to win an unprecedented third consecutive term, which the opposition have denounced as unconstitutional and a violation of the 2006 peace deal that ended a dozen years of civil war and ethnic massacres.

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

“Despite a facade of pluralism, this is an election with only one candidate, where Burundians already know the outcome,” said Thierry Vircoulon, an analyst with the International Crisis Group, which has warned the situation has all the ingredients for starting another civil war.

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

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 have 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 said on Monday that about a thousand people were fleeing each day to Tanzania, crossing the border “through the forest ... many traveling in the dark on foot and without belongings.”

In mid-May, rebel generals attempted to overthrow Nkurunziza. After that bid 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,” Leonce Ngendakumana, a prominent opposition figure, said after talks collapsed.

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

A poor and landlocked former Belgian colony, Burundi is in the heart of Central Africa's troubled Great Lakes region. 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 neighboring states, as fighting in the neighboring Democratic Republic of Congo has.

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

Al Jazeera and wire services