Karina said her unit was cut off from Farc's leadership
A top commander of the Farc rebels in Colombia has urged other rebels to follow her example and surrender.
Nelly Avila Moreno, known as Karina, handed herself in to soldiers over the weekend in the latest blow to Farc.
She said Farc was falling apart under pressure from the military and growing desertions. Several key leaders have been killed in recent months.
Karina has been blamed for a string of murders and abductions in the north-western Antioquia region.
Her surrender is a coup for President Alvaro Uribe, who made her a priority target for the security forces in 2002, the BBC's Jeremy McDermott says.
The Revolutionary Armed Forces of Colombia, or Farc, has been fighting to overthrow the government for more than 40 years.
"To my comrades: Change this life that your are leading in the guerrilla group and re-enter society with the government's reinsertion plan," she said at a news conference called by the army in Medellin.
Her unit had been whittled down to fewer than 50 fighters - down from several hundred - when she surrendered.
Karina said she had been out of contact with Farc's seven-member ruling secretariat for two years.
"The decision [to surrender] was made because of the pressure by the army in the area," she said.
She said she was shaken by the killing of secretariat member Ivan Rios by one of his bodyguards in March.
The bodyguard had cut off Rios's hand and turned it in with his laptop computer in return for a reward.
The government has offered bounties for top rebel commanders. Karina's was $1m (£512,000). Two weeks ago, President Uribe appealed to her to surrender.
She contacted the army who sent a helicopter to pick up her and another guerrilla, known as Michin.
She denied involvement in the 1983 murder of President Uribe's father and said she was not the "bloodthirsty" woman the authorities described her as.