Colombia's President declared the country's 50-year conflict with FARC guerrillas "truly over" Tuesday.
Santos himself shut a padlock on the last lot of decommissioned rifles before it was taken out of a remote demobilization camp to formally seal the UN-supervised disarmament by the Revolutionary Armed Forces of Colombia (FARC).
"With the laying down of arms ... the conflict is truly over and a new phase begins in the life of our nation," Santos said at a ceremony in Pondores, a remote area in the northern Guajira department.
"This is truly a historic moment for the country," he said.
"We have been a republic for 198 years. Never had we had such a long conflict and today is indeed the last breath of that conflict."
The leftist rebel force has said it will officially transform into a political party on September 1, a major step in reintegrating into civilian life as part of a historic peace deal signed last year.
"Soon we will be holding a founding congress for the new political party that will be called the Alternative Revolutionary Force of Colombia," said one of the FARC's senior leaders, Ivan Marquez, at Tuesday's ceremony.
The FARC was born in May 1964 from a peasants' revolt, and its ranks were made up mostly of country-dwellers who rallied behind the group's Marxist-Leninist ideology, with land reform its key demand.
As well as a quarter of a million dead, about 60,000 Colombians remain unaccounted for and seven million have been displaced in the conflict.