Burundi's army will absorb more than 2,000 former rebels
African Union troops are physically disarming 21,000 fighters from Burundi's last active rebel group, the Forces for National Liberation (FNL).
It follows a weekend ceremony where FNL leader Agathon Rwasa symbolically surrendered his own weapons to the AU.
A grenade attack killed six people but the BBC's Prime Ndikumagenge says it was not linked to the rebels.
But he says it shows how many weapons are circulating in Burundi following more than 10 years of ethnic conflict.
According to the AFP news agency, estimates put the number of weapons owned illegally at between 100,000 and 300,000.
National radio reported a mother and her five children were killed on Monday night when a grenade was thrown into a house in Buriri Province, south of the capital Bujumbura.
Officials say it is not known who the attackers were.
Our correspondent says the AU forces have deployed to five assembly points to collect the weapons.
The FNL was officially to become a political party at 1300 GMT, but the disarmament got off to a slow start in some areas and officials are still waiting to receive a report from the AU to proceed.
Returning home: 12,500
Some 3,500 combatants are to be integrated into the army and police.
Another 5,000 are to receive demobilisation packages - including a week's course to help them integrate into civilian life - 18 months' salary and $600 (£410) to start a business.
The rest - some 12,500 men - are being sent back to civilian life with an estimated $80 (£55).
Our reporter says at one of the assembly points 50km north-west of the capital, he saw more than 60 weapons - mainly AK-47s - being handed in.
But the rebels being sent home - some of whom have been fighting with the FNL for about 10 years - were complaining that $80 was not enough to help them rebuild their lives.
And there is concern amongst villagers around the capital that so many fighters are going home without receiving substantial support, he says.
But FNL leader Mr Rwasa told the BBC's Network Africa programme the country relied on donor funding and only had enough money to help so many fighters.
He also said ex-rebels would be considered first when it came to employing people for development projects.
Asked if he would ever take up arms again, he replied: "No.
"This decade of fighting is enough to teach every Burundian we have to refrain from whatever has been the cause of violence in the country."
The ethnic Tutsi minority has long held power in Burundi.
More than 300,000 people were killed in the civil war sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.
In the peace process that followed, former Hutu rebel leader Pierre Nkurunziza became president in 2005 - and posts in the previously Tutsi-dominated army have been split equally between Tutsis and Hutus.