The fighters from the "Heroes of Granada" faction of the United Self-Defence Forces of Colombia, or AUC, listened quietly as a trumpeter played taps to remember victims of paramilitary violence.
They then turned in their assault rifles and other weapons to government Peace Commissioner Luis Carlos Restrepo in Cristales, perched in the Andes 230km northwest of Bogota.
Under a contentious law passed by parliament in June, the government has granted reduced punishments and other concessions to warlords who demobilise their troops, confess their crimes and pay reparations to victims.
Most of the rank and file fighters have been granted complete amnesty.
With Monday's demobilisation, a total of 8500 AUC fighters have turned in their weapons in ceremonies that have taken place over the past two years.
But the process has come under criticism as being too lenient on former fighters who committed atrocities throughout the country.
On Monday, a Human Rights Watch report slammed the process and the new Justice and Peace law, saying it will leave the AUC's structure intact and not force leaders to return plundered goods.
The law "completely ignores the difficult - yet crucial - problem of how to dismantle the underlying structures and financial power of these groups", the rights group said.
The two main rebel groups have
not agreed to peace negotiations
The rights group also said the law provides no mechanism to ensure that demobilised fighters do not return to the war or to a life of crime.
The AUC is an umbrella group of outlawed paramilitary factions that sprang up two decades ago to wage a war against leftist rebels.
But critics charge that the warring sides have also become mafia-like organisations, kidnapping and carrying out extortion and controlling a large share of cocaine production in Colombia, the world's leading producer of the drug.
Murillo, who critics say only joined the AUC a few years ago to reap amnesty benefits, attended the ceremony on Monday to watch his fighters demobilise.
Getting off easy
US prosecutors, who have charged Murillo with drug trafficking, call him the de facto leader of the AUC.
Human Rights Watch said drug lords such as Murillo will very likely confess to drug crimes.
Under the peace deal, he could be given a reduced sentence and not face future trials in the US because a person cannot be charged twice for the same crime.
Even with the likely complete demobilisation of the AUC by year's end, Colombia's war will be far from over.
The two main leftist rebel groups, with more than 16,000 fighters in all, have not agreed to peace negotiations.