Bujumbura - Burundi is set to hold the presidential election on Tuesday amid unrest over incumbent President Pierre Nkuruniziza's bid for a third term in office.A grenade exploded in Bujumbura, the capital's central business district, but did not wound anyone, said Deputy Police Spokesperson Pierre Nkurikiye. The explosion highlights fears that the polls may trigger violence.Preparations for voting day are complete with ballot boxes and papers distributed to nearly 11 500 voting centres throughout the country and security has been improved to ensure a smooth voting process, the electoral commission Spokesperson Prosper Ntahorwamiye said on Monday.Ntahorwamiye said they expect a high turnout of the 3.8 million registered voters, saying that improved security has encouraged refugees to return. But the UN refugee agency said it has not seen any returns of the more than 170 000 who have fled the country fearing election-related violence.An uneasy calm has settled in Bujumbura, after weeks of sporadic street battles between the police and civilians. More than 100 people have died in demonstrations since the ruling party announced Nkurunziza's candidacy in April.Protesters say Nkurunziza must go because the constitution limits the president to two terms, but the president's supporters say he is eligible for a third term because he was chosen by lawmakers - and not popularly elected - for his first term in 2005.The weeks of street battles between police and civilians triggered an attempted military coup in mid-May that was quickly put down pro-Nkurunziza forces.Go homeGloria Ndayisaba 35, who runs a food kiosk in Musaga market, said she will not vote because Nkurunziza is on the ballot."Peter [Nkurunziza, as some refer to the president] should just go home and let us continue with our lives. It's time to let another person rule," Ndayisaba said, adding that her business was down by 50%.Dozens of Nkurunziza's critics including his second vice-president, the vice president of the electoral commission, and the deputy president of the Supreme Court have fled the country or are in hiding over death threats.Of the eight presidential candidates three, including two former presidents, withdrew from the race last week saying the polls will not be free and fair. Burundi's electoral commission has, however, denied receiving their withdrawals."Nkurunziza running for the presidency is a constitutional coup," said former president Sylvestre Ntibantunganya, who has pulled out of the race told The Associated Press on phone Monday.UN human rights chief Zeid Raad al-Hussein told the UN Security Council earlier this month that "the risk to human life, and to regional stability and development, is high" as a result of escalating politically motivated violence and Burundi's history of recurring bloodshed and atrocities.East African Community-backed negotiations between the government, opposition and other political actors aimed at ending the unrest stalled Sunday when government representatives did not attend.