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.
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.
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