Overnight attacks in Burundi killed a police officer and a civilian as the country opened polling stations for a controversial presidential election Tuesday. Gunfire and explosions could reportedly be heard in the capital Bujumbura Monday night. The country has been ravaged by violent attacks since President Pierre Nkurunziza announced in April that he would run for a third term.

Police battled gunfire with unknown assailants in the suburbs of Bujumbura, France 24 reported. In another attack, unknown passengers reportedly threw grenade on a street close to the Independence Square in central Bujumbura. There was reportedly no word on the casualties in either of the attacks.

Willy Nyamitwe, presidential adviser, held the opposition and protesters responsible for the pre-election violence. "People do it to intimidate voters. They don't want the voters to go to the polls," he told Reuters. About 3.8 million Burundians are eligible to vote in the presidential election.

In a statement released Monday, U.N. Secretary General Ban Ki-moon urged Burundians to maintain calm, and called to "refrain from any acts of violence that could compromise the stability of Burundi and the region."

Protesters allege that the 51-year-old president’s run for the third term is unconstitutional. Pie Ntavyohanyuma, head of Burundi's parliament, also called Nkurunziza’s third term “illegal.” He reportedly fled the country in June. The country’s constitution allows a candidate only two terms in power. However, Nkurunziza has denied the accusation by citing a constitutional court ruling.

Since April, at least 77 people have died and more than 127,000 people have fled amid violence in the East African country, Bloomberg reported.