Nearly 800 members of the main right-wing paramilitary group in Colombia are laying down their weapons in the city of Medellin.
The AUC has been fighting rebels in rural areas for two decades
The demobilisation by fighters of the United Self-Defence Forces of Colombia (AUC) is being seen as the first stage in a broader peace process.
The group has said its 13,000 men will all disarm by the end of 2005.
The paramilitaries have been a major force in Colombia's civil war since the 1980s, fighting left-wing rebels.
The AUC - like the leftist FARC and ELN - is on the United States' list of terrorist organisations. The Americans have repeatedly asked for the extradition of the group's leader, Carlos Castano.
Colombia's peace commissioner, Luis Carlos Restrepo, said he hoped the AUC demobilisation in Medellin would encourage other groups to follow suit.
But critics say the government is offering too many concessions to the paramilitaries.
The mass disarmament comes four months after the Colombian Government and the AUC agreed to start formal talks aimed at dissolving the paramilitary group.
It was the first time the government had agreed to negotiate with the AUC, which has been blamed for some of the worst atrocities in Colombia's civil war.
The demobilised fighters will spend the following three weeks in a government camp, receiving counselling and training.