At least nine Colombian policemen have been killed and six others injured by a roadside bomb in Santander province.
The authorities say it was planted by the Farc left-wing rebel group near Landazuri, 160km (100 miles) north of the capital, Bogota.
The policemen were taking part in a mission to destroy coca bushes, which supply the raw material for cocaine, a key source of income for the rebels.
Correspondents say it is the worst guerrilla attack this year in Colombia.
The dead were members of a police unit based in the north-eastern city of Bucaramanga.
Among the dead was the brother of top Colombian footballer Luis Yanes, who plays for Santa Fe in Bogota.
The attack is another blow for President Alvaro Uribe, who has vowed to crush the rebels by the end of his term in 2010.
Farc - the Revolutionary Armed Forces of Colombia - is the largest rebel group in Colombia. It has been fighting the government for more than four decades.