Burundian President Pierre Nkurunziza has been nominated as a candidate for a third term in office, a move that his opponents say is unconstitutional.
Security has been increased and demonstrations banned following opposition calls to step up protests.
Speculation about Mr Nkurunziza's candidacy has led to major protests in the capital Bujumbura in recent weeks.
There are fears the political crisis could destabilise a country only recently emerged from civil war.
The president's ruling CNDD-FDD party nominated him at a special congress on Saturday morning.
The presidential election is due to be held in June.
But the opposition says the move violates the country's constitution and a peace accord that ended the war.
The UN refugee agency says more than 8,000 people have fled to neighbouring Rwanda and the Democratic Republic of the Congo in recent weeks because of "pre-election violence and intimidation".
A former rebel leader, Mr Nkurunziza took office in 2005 following the end of a 12-year conflict, which killed more than 300,000 people.
He was re-elected in June 2010 but the vote was boycotted by the opposition, which complained of fraud in the earlier local elections.
Opposition leaders and international observers have since complained of a growing crackdown on opposition parties and the media.