Five opposition candidates have withdrawn from presidential polls in Burundi due to take place on 28 June.
They include the former rebel leader Agathon Rwasa, who was widely thought to be the key challenger to the current President Pierre Nkurunziza.
All had called for the resignation of Burundi's electoral commission following local polls last month, which they say were fraudulent.
But European Union observers have said the polls met international norms.
The governing CNDD-FDD won a comfortable majority in the local elections.
The BBC's Prime Ndikumagenge in the capital Bujumbura says the government has warned against any action that would undermine the country's safety during the elections.
The former candidates have called on their supporters to remain calm.
Some 300,000 people are believed to have died during Burundi's 12-year, ethnic-based civil war.
Mr Rwasa led his National Liberation Forces (FNL) in disarming in April 2009. It was the last active rebel group.
Mr Nkurunziza, also a former rebel leader, became president in 2005 following years of peace talks.
Both President Nkurunziza and Mr Rwasa led mainly Hutu rebel groups fighting against the army which was dominated by the Tutsi minority.