Bujumbura - A policeman and an opposition official died in violence at the start of Burundi's presidential election on Tuesday, held amid protests over President Pierre Nkurunziza's decision to run for a third term and an opposition boycott.Blasts and gunfire echoed around Bujumbura early on Tuesday in Burundi's worst crisis since a civil war ended in 2005. Calm returned to the capital during the day but dozens have been killed in weeks of demonstrations, a failed coup and clashes between rebel soldiers and the army.Voters queued outside polling stations in some rural areas and districts of Bujumbura that are strongholds of Nkurunziza supporters. In other areas there were only trickles of voters.Opponents accuse Nkurunziza of violating the constitution by seeking another five years in office. Nkurunziza, almost sure to win given the opposition boycott, cites a court ruling saying he can run again.Western donors and African states, worried about tensions in a region with a history of ethnic conflict, urged Burundi to postpone the poll. The United States and Europe have halted some aid to Burundi, one of the world's poorest nations.Presidential adviser Willy Nyamitwe said one policeman and a civilian had been killed in overnight violence. "People do it to intimidate voters. They don't want the voters to go to the polls," he told Reuters.Residents in the capital's Nyakabiga district identified a body found there as an official in the opposition MSD party and accused the government of killing him. A police source confirmed the victim was part of the opposition.A crowd blocked a thoroughfare with rocks and women chanted "We need justice and truth" near the body, before the Red Cross took it."We see the shooting last night as a kind of intimidation," said 32-year-old Desire Kabaya in Nyakabiga. "There will be chaos after this election because the government that will follow will not be recognised by all the people."Electoral commission chief Pierre Claver Ndayicariye told Reuters provisional results could be announced in two days. He said turnout could be 80% in rural areas but was unlikely to exceed 40% in the capital, the scene of protests. RefugeesThe US State Department said elections held in these conditions "will not be credible and will further discredit the government". It said Washington would review ties and could impose visa restrictions on those behind the instability.More than 170 000 Burundians have fled the nation of 10 million to refugee camps in Tanzania, Rwanda and Democratic Republic of Congo.Flanked by bodyguards jogging or walking alongside him, Nkurunziza cycled to a polling station in his northern home village of Buye, which was filled with soldiers.The president queued to cast his ballot, and before pedalling off, told reporters the election was to "allow the Burundian people to vote or to choose someone they believe in."In Bujumbura, Emery Ndayizere, 30, said there was no point voting. "The elections are just a masquerade because only one party is competing," he said.The electoral commission said opposition names were still on the ballots and any votes for them would be counted.Ferdinand, a 40-year-old voter in Bujumbura, said he would vote for Nkurunziza, a soccer fan who is often pictured rolling up his sleeves to apparently help people in the fields, because he had "a good programme of development for ordinary citizens."Opponents say the president's re-election bid undermines a peace deal that ended a civil war between rebel groups of the Hutu majority and the army, led then by the Tutsi minority.The tension worries neighbouring Rwanda, which has the same ethnic mix and suffered a genocide in 1994 that killed 800 000, mostly Tutsis as well as moderate Hutus.