Bujumbura - Polling stations opened in Burundi on Tuesday for a controversial presidential election, boycotted by the opposition and threatening to plunge the East African country into a deepening spiral of violence.President Pierre Nkurunziza's bid for a third term in office sparked months of demonstrations in which about 80 people have been killed.More than 160 000 Burundians have fled to neighbouring countries, with many of them saying they feared attacks by the ruling party's youth wing, the Imbonerakure. The army has meanwhile clashed with attackers believed to be renegade soldiers in the north.Protesters say Nkurunziza's bid violates the constitution and the 2000 Arusha agreement, which ended a 12-year civil war in 2005.The election was postponed twice, from June 26 to July 15 and 21, under pressure from the African Union, the European Union and the United States.Election-related violenceBut Nkurunziza ignored calls from Ugandan mediators not to stage the vote until the government had reached an agreement with the opposition. The government broke off talks with the opposition on the weekend, reportedly accusing it of supporting the insurgents.The UN Secretary General Ban Ki-moon said the dialogue was suspended "without agreement being reached on a range of issues that would have contributed to the creation of a climate conducive to the holding of credible and peaceful elections".The opposition boycott leaves only the ruling CNDD-FDD and three parties allied with it to compete for the presidency.About 3.8 million people have registered to vote. But many people were reported to be leaving Burundi for fear of election-related violence.A grenade exploded in Bujumbura on election eve. Witnesses said one person was injured.Up to 1 000 people are streaming daily into neighbouring Tanzania, where the Nyarugusu refugee camp is struggling to accommodate about 80 000 Burundians, according to the aid organization Medecins Sans Frontieres (Doctors Without Borders).Ban urged the authorities to "do all in their power to ensure security and a peaceful atmosphere during the election" and on all parties "to refrain from any acts of violence that could compromise the stability of Burundi and the region".