"We order all our units to cease any offensive military action against state armed forces or economic infrastructure, from 00:00 hours on Tuesday, May 20, to 24:00 on Wednesday, May 28," Xinhua quoted FARC spokesperson Ivan Marquez as saying in Havana, Cuba, where FARC and Colombian government officials are negotiating a peace deal.
Marquez described the gesture as a "beacon of hope" for a bilateral ceasefire, and an initiative that responds to a popular demand "that deserves to be heard".
The statement, read to reporters before delegates resumed their daily negotiations behind closed doors, was signed by the leaders of both guerrilla groups.
This is the third unilateral truce announced by the FARC since peace talks began in November 2012 to end a half century of armed conflict, and the first time both rebel armies have proposed a joint unilateral ceasefire.
The ELN is also looking to enter into peace talks with the government.
The government delegation in Havana, led by Humberto de la Calle, refused to comment on the guerrillas' announcement.