Weeks of unrest followed by President Pierre Nkurunziza announcement in April that he would seek another five years in office. He justified the move, which his opponents said was unconstitutional and foreign governments have criticized, on the basis of a favorable court ruling.
Almost 127,000 people, more than 1 percent of the population, have fled to neighboring states and more are leaving, many voicing worries about possible violence during voting, the U.N. refugee agency UNHCR said.
Announcing the boycott by the group of 17 parties of the July 15 presidential election as well as a parliamentary poll scheduled for Monday, Francois Nyamoya, secretary general of the MSD party, told Reuters that voting would "not be credible."
The dispute has plunged the poor African nation into crisis, raising concerns in a region with a history of ethnic conflict and leading some Western donors to scale back aid.
Protests erupted after Nkurunziza announced his re-election bid, leading to about six weeks of almost daily clashes between stone-throwing demonstrators and police, who were seen firing into the ranks of angry youths.
The streets of the capital, where most of the rallies were held, have calmed but political tensions remain. The election schedule has already been pushed back several weeks because of the unrest.
Frederic Bamvuginyumvira of the Frodebu party said his and other opposition parties opposed the new timetable that they said was drawn up without consultation, which the government denies.
He repeated calls for the ruling party's Imbonerakure youth wing to be disarmed, echoing comments from Western and African states. The ruling CNDD-FDD denies the youths have been armed.
One of Burundi's vice presidents, Gervais Rufyikiri, said this week he had fled the country after he was threatened for denouncing Nkurunziza's bid, an allegation denied by the government.
Government officials have repeatedly said they would guarantee a fair vote. When opposition parties threatened a boycott in the past, it said the ruling party's rivals were worried about losing at the ballot box and so wanted to stay away.
Opponents say the CENI election commission that will oversee voting is biased.
Two of the commission's five members resigned after the crisis erupted, including its vice president who fled Burundi. They have since been replaced.
UNHCR offices in the region "have been noting a steady increase in arrivals of Burundirefugees" as Burundi gears up for Monday's vote, UNHCR spokesman Adrian Edwards told a news briefing in Geneva today.
"People are telling us they are fleeing the general political instability, election-related violence, armed attacks and arbitrary arrests," he said, adding they included about 62,000 in Tanzaniaand 45,000 in Rwanda.
He said others were thought to have left but have not registered as refugees.