Colombia's largest left-wing rebel group, Farc, has admitted that it carried out the killings of 34 coca farmers earlier this week.
Cocaine is a vital source of income for the Farc
A Farc statement said the farmers supported right-wing paramilitaries and accused the government of shedding "crocodile tears over the deaths".
Tuesday's attack in Norte de Santander was the worst since President Alvaro Uribe took office two years ago.
The United Nations has condemned the massacre as a "war crime".
The UN High Commissioner for Human Rights in Colombia described the attack as "the premeditated murder of unarmed and totally defenceless civilians".
The farmers - who had been working at the farm for the past fortnight - were sleeping in hammocks, when the gunmen arrived at dawn on Tuesday in the village of Rio Chiquita.
Uribe came to power on a pledge to curb violence
They were bound hand and foot with ropes and shot with automatic weapons.
"We saved ourselves by running toward the mountain," Jesus Bayona, 45, who survived after being shot in the foot, told the AP news agency.
A regional police commander Lt Col William Montezuma later told the BBC that 50 gunmen took part in the attack.
A government official said the farm appeared to belong to right-wing paramilitaries.
Landless peasants often pick coca leaves and sell them to both left-wing guerrillas and their right-wing opponents, which can leave the farmers targets of reprisals.
Other villagers and farmers have been fleeing the area, fearful of further attacks, according to a human rights monitor.
President Uribe pledged to curb the violence which has plagued Colombia for decades, and increased military spending.