Up to 20,000 children fight in regional African conflicts
The 235 youths, aged between 15 and 19, had spent between three and five years as soldiers in the Congolese army.
The move comes amid reported progress in peace talks aimed at ending Congo's war.
International donors are meeting in Brussels on Wednesday to decide how best to respond to what is being described as one of the world's worst humanitarian crises.
Children release
The first group of children handed in their guns and uniforms at a military barracks in the capital Kinshasa.
"This is the effective start of the demobilisation of all child soldiers," President Joseph Kabila said during a ceremony at the Kibomango Training Centre.
The child disarmament programme was carried out jointly with the UN and the rebel group, the Congolese Rally for Democracy.
Unicef officials tried to qualm fears of uncertainty by the child soldiers.
They said the demobilised youths would attend a training centre where they are to be taught skills such as mechanics and carpentry to help them integrate into civilian life.
The Congolese Government says all 6,000 child soldiers will be demobilised.
The UN has previously expressed concern over the recruitment of children in the Congolese conflict and other regional battlefields, estimating that more than 20,000 are involved in fighting.
Poverty talks
The DR Congo has been involved in a conflict since 1998 when Rwanda and Uganda backed a rebellion against the late Laurent Kabila. The war has drawn five neighbouring nations into active combat.
But with signs of progress towards an end to the conflict, aid agencies have increasingly focused on Congo's desperate economic plight.
Last month, the UN said that more than two million war displaced people were in need of urgent life-saving assistance.
But the BBC's Mark Dummet in Kinshasa says international donors at the World Bank-sponsored talks in Brussels will be discussing whether this is the right time to help the DR Congo.
He says donors are split between those who think the aid should be stepped up before economic and social conditions collapse completely, and those who think it should be postponed until the end of the war and the announcement of elections.