President Juan Manuel Santos has warned that fresh violence could destabilize the demobilization of the leftist FARC rebels under a historic peace accord.
He signed the deal with the Revolutionary Armed Forces of Colombia (FARC) and pushed it through the legislature in December, defying criticism from conservative rivals.
In the weeks following, reports emerged of killings by local civil campaigners by unidentified groups in conflict areas.
Yesterday, the Victims' Unit, a state conflict resolution body, said in a statement that "17 civil leaders have been murdered since December 1, after Congress ratified the peace accord."
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Land rights were at the heart of the conflict that pitted the Marxist FARC against Colombian state forces since 1964.
The peace agreement reconciles the two main rival forces in the war, but there are fears of score-settling between renegade players in the multi-sided conflict.
As well as leftist rebels and state forces, the conflict drew in right-wing paramilitaries backed by landowners.
They were supposedly disbanded in the 2000s but the FARC and other groups say former members of them are still active.
"We are extremely worried by these events, because the truth is they are massacring social leaders," the unit's director, Alan Jara, said in a statement.
He called on prosecutors to investigate the killings and urged authorities to provide protection for social group leaders.