The ELN began fighting the government in 1964
A senior commander of Colombia's second-largest rebel group, the ELN, has broken out of prison after an armed raid by fellow rebels, officials say.
Gustavo Anibal Giraldo was being transferred to a court hearing when three gunmen on motorbikes attacked his eight-man escort.
One guard was killed and another hurt as the rebels overwhelmed the convoy in the north-eastern city of Arauca.
The ELN commander then fled on the back of a waiting motorcycle.
Police found a van and two motorcycles abandoned near the Venezuelan border shortly after Wednesday's attack.
Gustavo Anibal Giraldo Quinchia, better known by his alias Pablito, is believed to have fled to Venezuela.
Colombia's police chief announced a $830,000 (£518,000) reward for any information leading to his capture and asked Venezuela to co-operate in the manhunt.
Pablito is charged with kidnapping two US reporters and an American helicopter mechanic several years ago in Colombia. All three US nationals were later released unharmed.
The rebel leader was captured in January 2008, in what was seen as a heavy blow to the ELN (the National Liberation Army), the BBC's Jeremy McDermott in Colombia reports.
Pablito is regarded as one of the most aggressive and capable military leaders, and his escape could herald an upsurge in activity from the ELN, our correspondent adds.
The ELN has about 1,500 fighters, and - along with the larger Revolutionary Armed Forces of Colombia (Farc) - has been fighting the Colombian government since the 1960s.