The death toll in a weekend highway bombing in Colombia has risen to 20, with another 36 people injured, amid a surge of violence ahead of presidential elections next month.

Buses and vans were left mangled in the blast on Saturday on the Pan-American Highway, in the restive south-western Cauca department.

The governor, Octavio Guzmán, described the bombing as the area’s “most brutal and ruthless attack against the civilian population in decades”, adding that it left a crater 200 cubic metres in size. Several cars were flipped over by the force of the explosion.

Fifteen women and five men, all adults, were killed, he said. Of the injured, three people remained in intensive care. Five children were also injured but were “out of danger”.

The military chief, Hugo López, told a news conference on Saturday that the bomb had exploded after assailants stopped traffic by blocking the road with a bus and another vehicle. “It is a terrorist attack against the civilian population,” Lopez said.

The attack occurred just over one month before national elections, in which voters will pick a successor to the leftist president, Gustavo Petro.

“Those who carried out this attack … are terrorists, fascists and drug traffickers,” Petro said on X. “I want our very best soldiers to confront them.”

Petro blamed the bombing on Iván Mordisco, the alias used by the South American country’s most-wanted criminal, whom the president has compared to thee late cocaine kingpin Pablo Escobar.

Mordisco is the leader of a dissident faction of the now-defunct Revolutionary Armed Forces of Colombia (Farc) that operates in the region.

A bomb attack on Friday on a military base in Cali, Colombia’s third-largest city, injured two people and set off a string of attacks in the Valle del Cauca and Cauca departments.

According to López, 26 attacks have been recorded in the two departments over the past two days.

Authorities have boosted military and police presence in the areas, the defence minister, Pedro Sánchez, said Saturday.

Colombia has a history of armed groups – which finance their operations through drug trafficking, illegal mining and extortion – attempting to influence elections through violence.

Farc remnants who rejected a 2016 peace deal with the government have been actively trying to disrupt stalled peace talks with Petro.

Security is one of the central issues of the 31 Maypresidential elections. Political violence was brought into sharp focus last June, when the young, conservative, presidential frontrunner Miguel Uribe Turbay was shot in broad daylight while campaigning in the capital, Bogotá.

A leftist senator, Iván Cepeda, an architect of Petro’s controversial policy of negotiating with armed groups, is ahead in the polls.

He is trailed by the rightwing candidates Abelardo de la Espriella and Paloma Valencia, both of whom have pledged to take a hard line against rebel groups.

All three have reported receiving death threats and are campaigning under heavy security.