Farc leader Ivan Marquez said the group would start by making an inventory of its weapons before handing them over to United Nations monitors.
He said it was a sign of the Farc's "unconditional commitment to peace" after more than 50 years of conflict.
The government has yet to reach a peace deal with the ELN rebel group.
The Farc said its fighters would register their weapons with UN monitors and reveal the location of any stashed explosives.
Almost 7,000 rebels are now gathered in 26 transition zones where they are being registered and offered help to reintegrate into civilian life.
But it took months rather than weeks to get them there because of what the government said were logistical difficulties.
And when they arrived many Farc members complained the camps were unfinished and they had to build their own accommodation.
The Colombian government and the Farc insist that by 30 May - the date set in the peace accord for the disarmament process to be completed - all the guerrillas will be leaving the transition zones and entering civilian life.
But there have been serious delays, and the government and the rebels are trying to reassure Colombians that the peace process is still on track.
According to the original plan, the Farc were supposed to hand over 30% of their arms on 1 March.
But now only 320 rebels are set to disarm by the end of Wednesday, far short of the original target.
However, both the government and the Farc insist the disarmament will be completed in time and that they will meet the May deadline.
Colombia's Farc rebels say they will begin laying down their arms later on Wednesday as agreed in a peace deal signed in November.