At least seven people, including women and children, were massacred in the oil refining town of Barrancabermeja in north-eastern Colombia early yesterday, officials said.
The acting mayor, Mr Juan Carlos Sierra, said hooded and heavily armed gunmen dragged the victims out of their homes in a working-class neighbourhood and shot them.
The town, which has a history of violence, has long been considered a stronghold of Marxist guerrillas eager to maintain a foothold in the country's main oil centre.
More recently, ultra-right death squads - backed by the military, according to human rights groups - have moved in.
Mr Sierra said it was not clear who was responsible for Monday's killings. It was the worst mass murder in Barrancabermeja since June, when an ultra-right squad killed nine people.
In May 1998 a right-wing paramilitary force killed at least 32 civilians in a night of raids on two neighbourhoods of Barrancabermeja.
Amnesty International accused the military and police of backing that attack and at least three senior army officers were given a dishonourable discharge from the military in August for the role they played in permitting the massacre.