After a 64-year-old activist, Porofirio Jaramillo, was dragged from his home and murdered by four men on motorbikes, the head of the Colombian government’s victims unit did not mince his words. The country was facing a massacre in slow motion, he warned.

The attack on Jaramillo was the 17th murder of a community leader since Colombia signed a peace deal with the Farc (Revolutionary Armed Forces of Colombia ) rebels last December. The agreement won President Juan Manuel Santos a Nobel peace prize, for offering a country racked by half a century of war the prospect of security again.But for many activists, politicians and campaigners seeking to shape the new Colombia, peace has proved more dangerous than war, as they have been picked off by assassinations. The killings grew more frequent even as the government and guerrillas lurched slowly towards a deal, with about 100 murdered last year as the final negotiations were hammered out.

“We are extremely worried about these acts, because the truth is that social leaders are being massacred,” said Alan Jara, who heads the Victims Unit seeking reparations for the eight million people caught up in the war. “Their work is completely legal, and we should be providing them with state protection for their lives and conditions so they can continue with their work, in pursuit of better living conditions for their communities,” he told journalists after news of Jaramillo’s killing was reported.

A police officer guards the streets of the town of Pie de Pato, Choco, in western Colombia. Photograph: Luis Robayo/AFP/Getty Images

Violence is spreading particularly fast in areas abandoned by guerrillas under the peace deal because the promised state security forces have not arrived.

There is little clarity about who is behind the killings, but areas once held by guerrillas are often places where illegal mining or farming of coca – the raw ingredient for cocaine – make a tempting target for armed gangs or dissident rebels who refused to hand in their guns.

And the pace of the killings is feeding fears here that a government which battled long and hard for a peace deal is being dangerously lax about making paper plans a reality.

“They are leaving a power vacuum, even though this was anticipated. The government said they were going to move the armed forces into these zones and they haven’t. And the people in these areas are very concerned,” said Marc Chernick, director of the centre for Latin American studies at Georgetown University in Washington.

“This is critical to consolidating the peace agreement. If they can’t bring control and stop the assassinations it’s going to be very hard for this process to move forward. I think everyone understands this, but yet we are not seeing the government responding adequately.”

Hanging over the country is the memory of another peace deal and another massacre 30 years ago. Around 3,000 members of the leftwing Unión Patriótica were murdered, effectively sinking that agreement and leaving a deep scar on the national psyche.

Either everything that sounds so good on paper must be put into practice, or it will be too late for many areas, where a new enemy is lurking Foundation for Peace and Reconciliation

Human rights groups are certain the killings are intended to “intimidate society as the peace process advanced in Colombia, and political alternatives increase,” the Centre for Historical Memory noted in a response to a round of killings in November, including a grandmother gunned down on the school run.

The headline of their statement bluntly spelt out worries that another peace deal could be at risk. “Don’t let a genocide like the UP killings happen again,” it warned.

At the heart of much of the violence are land rights and the country’s multi-billion-dollar cocaine industry, which for years helped fund the Farc. During the years of war large swaths of land were seized by ranchers, drug traffickers and armed groups themselves, who have little interest in relinquishing it.

The peace deals envisages the dismantling of the drugs trade, with the Farc helping persuade farmers in areas it once controlled to switch to other crops, and the return of land to farmers who once owned it.

But armed criminal groups are eager to take over the trade where the government does not replace Farc soldiers rapidly enough. Some are already reportedly trying to sign up former guerrillas with monthly salaries three times their government reintegration stipend.

In one corner of the country, Chocó, the security vacuum had such devastating consequences that in November residents begged the country’s only remaining guerrilla group, the ELN, to take control of the area left by the Farc, Spain’s El País newspaper reported.

The Catholic church has also weighed in. Earlier this month the bishop of Apartado, a town on the north coast, put out a fierce statement warning the government that whole swathes of the country were at risk of spiralling out of control.

“We can see the region is being consumed by forced displacement, drug trafficking, youth gangs, environmental destruction and an increase in illegal mining,” Bishop Hugo Alberto Torres Marín said.

“We cannot forget what has happened in the past and threatens to repeat itself, the tragic history of systematic assassination of so many people committed to bringing social and political change.”

The government has promised to send nearly 70,000 troops to areas once held by the Farc. But it is still not clear when they will be deployed and in the areas at risk there is a sense that time is running out.

“Ultimately the problem is one of time, and a question of who has more initiative. While the government forces are adapting to the new situation, armed groups are in an atmosphere conducive to growing their thriving illegal businesses,” the Foundation for Peace and Reconciliation said.

“Either everything that sounds so good on paper must be put into practice, or it will be too late for many areas, where a new enemy is lurking.”