At least 2,000 combatants and a key leader of Colombia's main right-wing paramilitary group, the AUC, have surrendered their weapons.
The fighters are believed to belong to the AUC's biggest faction
They handed over thousands of firearms, two gunship helicopters and several vehicles in exchange for an amnesty.
The ceremony took place at a heavily guarded ranch in northern Colombia.
The move marked a return to the peace process between the AUC and the government of President Alvaro Uribe.
The paramilitaries halted disarmament in October, fearing the authorities planned to extradite one of its leaders to the US.
Colombian Peace Commissioner Luis Carlos Restrepo attended the ceremony at the Bellavista ranch, in the Antioquia department.
The camouflage-clad fighters were all members of the Central Bolivar faction of the United Self Defence Forces of Colombia (AUC), which is reportedly the biggest, with up to 8,000 fighters.
Their leader Ivan Roberto Duque, known as Ernesto Baez, and commander Carlos Mario Jimenez, alias Macaco, also surrendered their weapons.
Under the terms of the controversial two-year peace process, those who have committed crimes face reduced prison terms.
Most of the rank-and-file paramilitaries will be given a full pardon and can be eligible for job-training programmes and a monthly government stipend for two years.
Human rights groups say the process will merely "recycle" fighters and allow them to act with more impunity.
The authorities say that nearly 10,000 of the 20,000 fighters in the AUC have disarmed under the peace process.
Meanwhile, the Colombian government is also due to begin talks this week in the Cuban capital with the leadership of the second biggest left-wing rebel group, the National Liberation Army (ELN).
The ELN and the larger Revolutionary Armed Forces of Colombia (Farc) have been involved in a 40-year conflict with state forces and right-wing fighters.