Right-wing paramilitaries killed as many as 30 farm workers in southeastern Colombia and 10 fishermen in the north, many of them shot while lying face-down on the ground, authorities said.
Left-wing guerrillas, meanwhile, announced the execution of two police officers they had kidnapped over the weekend, throwing the tenuous three-year peace process into further turmoil.
The Armed Forces Commander, Mr Fernando Tapias, said 17 people were confirmed dead in two attacks on Wednesday in the southwestern township of Buga, but reporters on the scene, some 400 km south-east of Bogota, placed the number of dead as high as 30.
The reporters said the attackers identified themselves as members of the United Self-Defence Forces of Colombia (AUC) right-wing paramilitary group, believed by many, including human rights advocates Human Rights Watch, to have tacit support from the Colombian military.
The AUC and its sworn enemies, the left-wing rebel Revolutionary Armed Forces of Colombia (FARC) and the National Liberation Army, murder civilians they believe support their opponents. The US-based Human Rights Watch earlier this month reported that Colombia's "military and police continued to work with and tolerate paramilitary groups responsible for the country's most serious human rights violations".
A local official of the Valle department, where Buga is located, said the massacres took place in the nearby villages of La Habana and Alaska.
Witnesses said the gunmen took several families out of their homes at gunpoint, while another group was forced off buses in the area. They then separated the men from the women and children and took them to different places, made them lie face-down on the ground and riddled them with bullets.
The AUC was also blamed for the murder of 10 fishermen on Wednesday near Cienaga, a town in the Magdalena department, 950 km north of Bogota, the Attorney General's office said.
FARC said it had killed two police officers it kidnapped over the weekend along with four other people on a highway 700 km south-east of Bogota. The two police officers were killed by a rebel firing squad for allegedly belonging to a right-wing paramilitary group, a rebel spokesman said. The announcement caused anger among officials as FARC told government negotiators they would stop mass civilian kidnappings. The Ombudsman, Mr Eduardo Cifuentes, said the rebels were jeopardizing the peace talks.
FARC guerillas yesterday freed the last two of three German men whose kidnappings in July stained fragile peace talks, a Red Cross spokesman said last night. Mr Ulrich Kunzel and Mr Reiner Bruchmann, who were kidnapped together with Mr Kunzel's brother, Thomas, were released unharmed in the southern province of Cauca.