Burundi's main opposition party has picked Agathon Rwasa as its candidate for the country's May 20 presidential election.

Members of the National Congress for Liberty, known by its French acronym CNL, approved the 56-year-old's nomination on Sunday, the party announced on Twitter.

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A former rebel leader and longtime political opponent of outgoing President Pierre Nkurunziza, Rwasa was the leading opposition candidate in two previous elections in 2010 and 2015 - but boycotted both of them.

In 2015, Nkurunziza's controversial decision to seek a third term plunged the country into its worst crisis since the end of a bloody civil war a decade earlier, with rights groups saying hundreds of people were killed in a crackdown by the security agencies on protesters in the months that followed the president's re-election.

At the time, the opposition had accused Nkurunziza of violating the constitution by seeking another term. The president cited a court ruling saying he could run again.

Rwasa will run in the upcoming polls against army General Evariste Ndayishimiye, an ally of Nkurunziza who was chosen last month by the ruling CNDD-FDD party to be its candidate. On Sunday, the opposition candidate denounced what he alleged were plans to rig the vote.

"As we are approaching elections, it's surprising to hear that there are people thinking about rigging elections," he told delegates of his party after his appointment was announced. "Burundians will not let them do it."

The United Nations has warned that human rights abuses might increase again before the elections.

Nkurunziza was widely expected to take advantage of recent constitutional changes adopted by a referendum to stand for re-election, raising concerns that Burundi would see a repeat of 2015's deadly unrest.

Last month, the country's parliament overwhelmingly approved a bill granting outgoing presidents lavish send-off perks including a luxury villa and a one-off sum equivalent to more than $500,000.

The amount is a fortune in Burundi where more than 65 percent live in poverty and where 50 percent of the country is food-insecure, according to the United Nations's World Food Programme.