The head of Colombia's Revolutionary Alternative Forces of the Commons (FARC), Rodrigo Londoño, is urging former guerrillas to remain calm and refrain from violence following the murder of former leader Juan Vicente Carvajal Isidro.
Londoño called on ex-militants not to jeopardize the fragile peace agreement between the government and the once-armed group turned political party.
"Discipline was always necessary for the preservation of life in war," he said, urging former combatants to resist acts of violence, kidnapping or revenge.
"The conflict did not end, but changed its form of expression, which requires our consistent good behavior; our strength lies in the collective, we cannot forget."
Carvajal Isidro had surrendered his weapons along with 7,000 former guerrillas last year, after more than half a century of armed conflict.
After quitting the Training and Reincorporation Agreements (ETCR) state facility, he purchased land and opened a nightclub in an effort to "form a new identity in society," Londoño said.
He was attacked and killed on Monday in Arauca near the Venezuelan border, authorities report.
"Enough bureaucratic excuses for not complying with the provisions... in terms of protection, legal security and comprehensive reincorporation," Londoño said.
"The recent attacks and threats against the militants of our party in the cities of Bogota and Pereira... as well as the presence of paramilitary structures in various departments of the country, must force the authorities... to take effective and appropriate measures against criminal structures."
At least 22 former FARC members, along with some 40 relatives and associates, have been murdered so far this year. Eight of these have been attributed to paramilitary and dissident attacks.