The formal talks, which were set to begin Sunday in the eastern border town of Tibu, will instead start October 16, said representatives for Colombian President Gustavo Petro and the rebel group known as the Central General Staff (EMC).

Presidential peace advisor Danilo Rueda said the ceasefire, announced last month, would also now come into effect on October 16.

His remarks were met with jeers among the thousands of Indigenous and other rural inhabitants who traveled to Tibu to witness the opening of negotiations.

Tensions have mounted in recent weeks as preparations for the talks were carried out, with clashes between guerilla forces and Colombian troops resulting in at least 22 people killed.

Petro, Colombia's first-ever leftist president and a former guerilla himself, has sought to give the dissidents a second chance to lay down arms after they rejected a historic peace agreement in 2016.

The 2016 deal saw about 7,000 members of the Revolutionary Armed Forces of Colombia (FARC) give up arms and attempt to reintegrate into civilian life, though a faction under guerrilla leader Ivan Mordisco decided to keep fighting.

Petro took office last year vowing to bring "total peace" to a country battered by decades of civil conflict between the state and various left-wing guerrilla groups, right-wing paramilitaries and drug traffickers.

The EMC -- which had about 3,500 members by the end of 2022 according to official figures -- steadily increased its presence in territories formerly occupied by the FARC and largely abandoned by government forces.

It is said by the government to be involved in cocaine trafficking, illegal mining and attacks on Colombian troops.

EMC spokesperson Andrey Avendano, who received applause from the audience, acknowledged that the road to peace with the government will be long.

"We are fully convinced that it is through dialogue, that it is through a political solution that we are capable of resolving the different problems," he said.

Petro's government has also been negotiating since November with another group, the National Liberation Army (ELN) guerrillas, in rotating venues between Cuba, Mexico and Venezuela.