Three opposition candidates, including two former presidents, have announced their withdrawal from Burundi's upcoming presidential elections.

Domitien Ndayizeye and Sylvestre Ntibantunganya - former presidents - and Jean Minani, former speaker of parliament, said in a letter sent to the electoral commission on Saturday that the political and security environment could not guarantee free and fair elections.

The letter announcing the pullout from next Tuesday's election is the latest blow to the country's political system, which has been rocked by months of violence after the incumbent President Pierre Nkurunziza announced his bid for a third term - a move critics are calling unconstitutional.

Opposition leaders have complained that they have been unable to campaign because of intimidation by government security agents.

Nkurunziza chose to spend Saturday, the last day of the campaign, in his hometown Ngozi, watching a football match between local teams.

Prosper Ntahorwamiye, the commission spokesman, said on Saturday he had received the letter but the names of the candidates will remain in the ballot papers which had already been printed and distributed.

Burundi has been tense since April when the ruling party nominated Nkurunziza to be its candidate - prompting widespread demonstrations in the capital, Bujumbura.

On Saturday, the the AU Commission said in a statement that it was ready deploy human rights observers and military experts to Burundi.

"The Commission will initiate the required formalities from Monday 20 July 2015, and will commence the deployment as soon as the visas have been issued," the AU Commission said.

At least 77 people have died in the sporadic demonstrations, which at one point boiled over into an attempted military coup in May that was quickly crushed by pro-Nkurunziza  forces.

The opposition says Nkurunziza must go after serving his two constitutionally allowed terms.

The nation's constitutional court has ruled in the president's favour, however, maintaining he is eligible for a third term because he was chosen by legislators - and not popularly elected - for his first term.

Nearly 144,000 refugees have fled Burundi since April fearing election violence. Burundi's government and opposition groups are in negotiations aimed at ending the political unrest.

Source: Agencies