Colombia's Farc rebels have begun an indefinite, unilateral ceasefire.
The group unexpectedly announced on Wednesday that it would begin a truce for an unlimited time from Saturday.
The move has been welcomed by the UN and the European Union. However, Colombia's government said it would not join the rebels in the truce.
Hours ahead of the ceasefire, the army said Farc had killed five soldiers in an ambush. The rebels attacked a patrol in a rural area of western Colombia.
President Juan Manuel Santos described Farc's ceasefire declaration as a "gift... full of thorns".
He has rejected rebel calls for a bilateral truce, warning that this would give them the chance to re-arm.
He also condemned the ambush, saying the soldiers had died "defending the security of their fellow Colombians".
Representatives of Colombia's government and the Farc have been holding talks in the Cuban capital, Havana, for more than two years.
The talks were almost derailed in November after the Farc captured a Colombian general, Ruben Dario Alzate, prompting President Santos to suspend the negotiations.
The rebels released the general unharmed in an effort to revive the talks.
Earlier this week, they announced that they would observe a truce starting at midnight local time (0500 GMT) on Saturday. It should become a formal armistice and would only end if they were attacked, they said.
The Farc have declared ceasefires in the past, but these have been temporary. It has also staged similar attacks to the one on Friday, just before the commencing its ceasefires.