Colombia has previously accused Hugo Chavez of funding Farc
Venezuelan President Hugo Chavez has called on Colombia's Farc rebels to end their four-decade struggle and release all their hostages.
Mr Chavez, whom Colombia has accused of financing the Farc, said they were "out of step" and their war was "history".
The Colombian government expressed surprise, but welcomed the statement.
The rebels are believed to be at their weakest point in years, following the death of their long-time leader, Manuel Marulanda, in March.
The Revolutionary Armed Forces of Colombia (Farc) hold many hostages, including about 40 high-profile ones they say they want to swap for imprisoned rebels.
In his weekly television and radio programme on Sunday, Mr Chavez urged the Farc's new leader, Alfonso Cano, to "let all these people go".
"There are old folk, women, sick people, soldiers who have been prisoners in the mountain for 10 years," he added.
The Venezuelan president said ending the rebellion could lead to a peace process between the rebels and the Colombian government.
"The guerrilla war is history," he said. "At this moment in Latin America, an armed guerrilla movement is out of place."
Colombian Interior minister Carlos Holguin said the statement from Mr Chavez, a "great ally" of the rebels, was "surprising".
"He is a great defender and ally of the guerrillas, so it is so surprising," he said.
"But it's great, and I hope Farc hears him."
BBC Americas editor Emilio San Pedro says the message represents an about-face for Mr Chavez, who a few months ago called on the world to regard the Farc as a legitimate army rather than a terrorist group.
Our correspondent says Mr Chavez' critics will wonder whether this change is related to allegations by Colombia's Alvaro Uribe - who accused Mr Chavez of giving the rebels $300m.
In March Colombia said it had found documents on a computer that proved Venezuela funded Farc.
The computer was seized during a raid on a Farc camp in Ecuador, in which another senior Farc leader, Raul Reyes, was killed.
Venezuela said any contacts with Farc were solely made as part of a humanitarian effort to free hostages.
The captives include Franco-Colombian politician Ingrid Betancourt and three Americans.
Earlier this year Mr Chavez negotiated the release of two key hostages, Clara Rojas and Consuelo Gonzalez.
Late last year his official mediation role was terminated by Colombian President Alvaro Uribe, who said Mr Chavez had overstepped his responsibilities.
The Farc is the oldest and largest group among Colombia's left-wing rebels - and is one of the world's richest guerrilla armies.
It was founded in 1964 when it declared its intention to use armed struggle to overthrow the government and install a Marxist regime.
But like most of the paramilitary groups in Colombia's forty-year civil conflict, it has become increasingly involved in the drug trade.
Colombia is known as the "kidnap capital of the world", with one person a day, down from 10 a day in 2002, being snatched either for ransom or political bargaining.