The army blamed Marxist rebels of the Revolutionary Armed Forces of Colombia (FARC) for the blast on Saturday.

The car that blew up was said to have been packed with 20 kg of an explosive used in the mining industry.

The Marxist rebels are also blamed for Thursday's explosion in the city of Medellin which killed four people, and Friday's aborted attempt to blow up a truck near Bogota.

Violent anniversary

The armed forces said the FARC was intent on marking the 40th anniversary of its creation on 27 May, with a wave of violence.

FARC has evolved from a band of peasants struggling for land reform into a 17,000-strong force funded by kidnapping and "taxing" the cocaine industry.

But a strengthened military, supported by President Alvaro Uribe, has pushed the FARC deeper into its jungle hide-outs.

The number of FARC attacks had fallen sharply since a high point several years ago when the rebels were capable of overrunning army posts.