The Colombian Government and the main right-wing paramilitary group have agreed to start formal talks aimed at disarming about 10,000 gunmen.
The AUC has been fighting rebels in rural areas for two decades
In a joint statement, the government and the United Self-Defense Forces of Colombia (AUC) said all of the group's fighters would begin disarming at the end of the year, in a process expected to last until the end of 2005.
The paramilitaries have been a major force in Colombia's civil war since the 1980s, fighting left-wing rebels groups.
The move was announced as President Alavaro Uribe temporarily relocated to one of Colombia's worst war zones, to demonstrate his determination to restore the state's authority.
The agreement with the AUC marks the first time the government has said it would negotiate with the group - which has been blamed for some of the worst atrocities in Colombia's civil war.
The AUC - like the leftist FARC and ELN - is on the United States' list of terrorist organisations, and the Americans have repeatedly asked for the extradition of its leader, Carlos Castano.
"Their moment in history has ended... They are undesirable for the country, and the moment for their demobilisation has come," said chief government negotiator Luis Carlos Restrepo.
The joint statement said the authorities would help integrate former paramilitary fighters into civilian life.
The AUC is an umbrella group for many of the paramilitary forces that arose to counter violence by leftist rebels in rural areas, where government troops had little or no control.
Security is tight in Arauca for President Alvaro Uribe's stay
One such region has been the north-eastern state of Arauca, where the president transferred the government on Tuesday.
"This is a symbolic act to take the presence of the government to all corners of the country," a presidential aide told AFP news agency.
"The president wants to show that he can govern the whole territory," he added.
Paramilitaries, the FARC and the ELN are all active in the region.
A car bomb killed a policeman during President Uribe's last visit to the city of Arauca in October.
The government is due to be based there for the next three days.
Ahead of the president's arrival troops combed the city, searching for weapons and explosives.