The leader of Colombia’s revolutionary guerrilla movement, the Farc, has pledged to maintain the fragile ceasefire that has halted the world’s longest-running civil war even if the country’s peace deal is rejected in a plebiscite on 2 October, he has told the Observer.

On Sunday in the Caribbean city of Cartagena, the president of Colombia and the leader of Latin America’s most enduring guerrilla insurgency will put their signatures on the agreement that ends more than 50 years of fighting.

The leader of that insurgency by the Fuerzas Armadas Revolucionarias de Colombia (Farc) is Rodrigo Londoño Echeverri – known by his nom de guerre Timochenko. He told the Observer in an exclusive interview that, even if the accord is not ratified, there will be no return to war. Timochenko said, as the negotiating teams of Farc and President Juan Manuel Santos closed in on the peace deal: “All Colombians have felt the pain, for one reason or another – the pain of a mother guerrilla fighter is the same as the pain of the mother of a soldier. We are from the same fatherland, so we cannot use this process to throw salt into the wounds of the other side. It’s about how we agree to heal those wounds, not repeat them. We cannot sow the seeds of another conflict.”

Santos has warned: “If ‘No’ wins, we will return to what we had at the start of this government six years ago. We return to armed conflict. That would be a catastrophe for the country.”

Polls vary on the outcome of next Sunday’s referendum – most, but not all, showing a majority for the government peace plan, but not a large one. Hardline former president Álvaro Uribe leads a movement opposed to the accord, which he calls an amnesty for terrorism.

The Colombian peace process has been four years in the making, across a negotiating table in Havana, Cuba, with the governments of Raúl Castro, and that of Norway, acting as guarantors. Last week at a Farc camp called El Diamante – deep in jungle in the south of the country and visited by the Observer – the guerrilla organisation held a 10th congress at which it voted to abandon the armed struggle and dissolve itself as a military organisation after 52 years of fighting.

In addition to surrendering its arms, the Farc must assist the government in an agreement to work jointly towards eradication of coca crops. The accord commits the government to rural reform and protecting the Farc from rightwing militias as it forms a new political party, and both sides to co-operation with a truth commission and transitional justice mechanism.

Facebook Twitter Pinterest Farc soldiers playing football during the organisation’s conference on the peace agreement. Photograph: Mauricio Duenas Castaneda/EPA

The Observer has, over the past year, gone deep within this last enduring Marxist insurgency, visiting its remote rainforest redoubt and interviewing all five leading Farc commanders in Havana for a film.

In the jungle, entire encampments have been abandoned, bomb shelters are empty, guns – soon to be decommissioned – hang idle next to metal water-bottles and civilian clothes and field hospitals have no patients.

One Farc guerrilla, Antonio, told the Observer: “We can’t go back to war. We don’t want to die, no one wants to die.” And another, Duan, said: “We have fought for so long, for our rights and for the poor, but we have families and we are ready to return to normal life.”

Commander Carlos Antonio Lozada – the man likely to lead the Farc into elective politics – said that during the peace talks “both sides faced the same risks, the same situations. We started to get to know each other and we found out that we speak the same language.”