Social and campesino organizations are calling for the Colombian the Prosecutor's Office to send a commission to verify victims' testimony following Thursday's violence in Tucamo which left several people dead and dozens injured.
They also want the Attorney General, the United Nations and the Ombudsman's Office to take part in the probe.
An official government investigation is already underway.
The incident occurred in Tumaco, Narino, in the southwest of the country, as coca growers were protesting the forced eradication of their crops by the government.
Local reports and campesinos' rights groups say state security forces opened fire on the demonstrators.
The exact number of dead is in dispute.
Social organizations say nine people were killed and 50 wounded.
While the Proscecutor's Office puts the official figure at six dead and 30 injured.
At a news conference, the Social leader Luz Perly Cordoba of the National Coordinator of Coca, Poppy and Marijuana Growers, COCCAM, demanded "a verification mission and truth commission to clarify violations of human rights by the public force ".
David Florez, of the Patriotic March rights group, told reporters the incident in Tumaco was a "massacre against the campesinos. We demand a change in command of the police in the area."
Florez insisted two commissions are needed. The first must "go there quickly, take photos, collect testimonies and provide an initial report." The other should focus on "technical conditions that allow them to start a formal part of the process," to tell " step by step what is known, because this incident must be rejected by all the Colombians."
Police have said dissident FARC rebels who have rejected the peace process in Narino, one of the areas hardest hit by state and drug trafficking violence, were responsible for the unrest.
The Daniel Aldana Mobile Column has been accused by the Colombian Defence Ministry of using explosives and firearms to attack civilians as well as police and military forces sent to manually destroy coca crops.
But witnesses say officers and the army fired on the protesters. One told the media "We were arriving in a crowd full of people and the police fired at us, I saw the policeman about 50 meters away and he shot me and wounded my arm, it was not dissidents who shot us."
On Friday, Colombia's President Juan Manuel Santos failed to address the campesinos' claims and instead blamed the incident on criminal gangs involved in drug trafficking.
Bandas criminales se oponen a erradicación forzosa y presionan a campesinos para que no sustituyan. Por estos hechos serán judicializados.— Juan Manuel Santos (@JuanManSantos) October 6, 2017
Santos also noted that it was a FARC territory, without explicitly blaming the former rebel group which has now transitioned to civilian life and a political party.
He acknowledged the security forces’ version had been challenged and said this would be investigated.
“We have a number of different versions of the story that we’re confirming,” the president said. “So for now we’re not confirming one way or the other what happened.”
Santos insisted, however, that the massacre had not been committed by police and announced an increase in the rewards for dissident FARC commanders “Guacho” and “David” who are active in the area.
According to the Afro-Colombian Community Council of Alto Mira and Frontera, local rural communities have been under increased pressure from armed groups seeking to use civilians as "human shields" against government eradication activities.
Bogota is striving to achieve its goal of eradicating 100,000 hectares of coca by the end of this year.
The incident has received international condemnation.
The head of the U.N. Special Mission in Colombia Jean Arnault said he “profoundly laments what happened ...in Tumaco” and offered condolences to the victims’ families.
Dozens of parliamentarians in the U.K. expressed “serious concern” over the “actions of the Colombian Public Forces in Tumaco” that left “a thirteen-year-old child” among the dead.
And the human rights advocacy group, Washington Office on Latin America, condemned the “massacre against civilians” in Tumaco as “inexcusable.”