A bomb attack on a passenger bus travelling along the Pan-American Highway in southwestern Colombia has killed at least 20 people and injured dozens more, in the deadliest incident of a surge in attacks that has gripped the region ahead of next month's presidential election.

The explosion occurred on Saturday in the municipality of Cajibio, in the Cauca department — a restive region in Colombia's southwest long contested by illegal armed groups. An explosive device was detonated as the bus moved along the highway, leaving vehicles mangled and blowing a large crater in the roadway. Several cars were overturned by the force of the blast. Among the 20 confirmed dead were 15 women and five men; 36 others were injured, including five minors and three people left in intensive care. Colombia's Institute of Legal Medicine deployed forensic specialists, including dentists and anthropologists, to help identify victims. The governor of Cauca, Octavio Guzmán, declared three days of mourning.

Military chief General Hugo López described the attack as a "terrorist act" and said assailants had first blocked the road with a bus and another vehicle before detonating the bomb. He attributed the attack to the network of a fugitive known as Iván Mordisco and the Jaime Martínez faction — both dissident offshoots of the now-defunct Revolutionary Armed Forces of Colombia, or FARC, the guerrilla organisation that signed a peace deal with the government in 2016 but whose splinter groups remain active. President Gustavo Petro publicly blamed Iván Mordisco, comparing him to the late cocaine kingpin Pablo Escobar. The bombing was part of a broader escalation: López said more than 26 attacks had been recorded across Cauca and the neighbouring Valle del Cauca department over just two days, including a Friday bomb attack on a military base in Cali, Colombia's third-largest city. The United Nations High Commissioner for Human Rights condemned the attacks on civilians and called for a thorough investigation.

The violence carries particular weight coming just over one month before Colombia's May 31 presidential election, in which voters will choose a successor to President Petro. Security has emerged as a defining issue of the campaign, sharpened by last June's assassination of a young conservative frontrunner who was shot while campaigning in the capital, Bogotá. All three leading candidates — leftist Senator Iván Cepeda and right-wing contenders Abelardo de la Espriella and Paloma Valencia — have reported receiving death threats and are campaigning under heavy security. The Cauca region, where coca leaf cultivation and drug trafficking routes to Central America and Europe make it a prize for armed factions, has for decades been one of Colombia's most conflict-affected areas. Defence Minister Pedro Sánchez said authorities have boosted military and police deployments in response to the latest wave of attacks.