The 2006 peace deal has taken three years to implement
The disarmament of Burundi's last major rebel group, Forces for National Liberation (FNL), has begun after months of delays.
Some 3,000 singing and chanting former rebel fighters began gathering late on Monday at an assembly site in the west of the capital, Bujumbura.
According to a peace deal reached last year, the ex-fighters will now be integrated into civilian life.
The FNL is to form a political party before the elections due in 2010.
"We are happy because we have arrived here. We have been waiting for so long to come. Now we have left the bush," one ex-fighter said.
The exercise was initially due to have been completed by the end of 2008.
The mainly Hutu rebels, government and African Union peacekeepers have all blamed each other for the delay.
And as the process started, deputy FNL chairman Alfred Bagaya questioned when all 19,000 combatants could gather, saying the designated area could only hold 6-8,000.
Ex-rebel leader Pierre Nkurunziza was elected president in 2005 under a deal to end years of conflict between the Tutsi-dominated army and mainly Hutu rebels.
About 300,000 people were killed in more than a decade of civil war.