Colombian officials have shown videos and photos seized from captured rebels of 16 high-profile hostages, the first sign of life since 2003.
Among those on the five tapes are French-Colombian politician Ingrid Betancourt and three US nationals.
Officials said the video of Ms Betancourt, who has been held for five years, dated from last month.
French President Nicolas Sarkozy said France would "redouble efforts" to secure Mrs Betancourt's freedom.
"I have always said we will never forget Ingrid Betancourt. We know she is alive. Now we have to fight with all our might to free her from her ordeal," Mr Sarkozy said.
Mr Sarkozy has made securing Ms Betancourt's release a priority of his administration.
"We are extremely moved, very, very moved to see these images of my sister," Astrid Betancourt told French television.
"It is a sad image of my sister but she is alive."
Ms Betancourt's son Lorenzo called for a humanitarian agreement to get the hostages out.
"I felt relieved first (when I saw the video) but I also have a feeling of anger to see in her in this state, to see that she is skinny, that she is weak, " he said.
"She's been there for six years, and I think she won't cope in this state for much longer."
The videotapes, which were without sound, were played at a news conference and shown on Colombian television.
One tape showed Ingrid Betancourt, looking extremely thin, sitting on a chair in a jungle setting, not speaking but just looking at the ground.
Ms Betancourt, who has dual French and Colombian citizenship, was kidnapped in 2002 as she campaigned for the presidency.
Other tapes showed American defence contractors Thomas Howes, Keith Stansell and Marc Gonsalves, who were seized by Farc guerrillas after their plane went down in southern Colombia in 2003.
The men were also shown against a jungle backdrop but appeared in better health than Ms Betancourt.
There were also videos showing images of 12 Colombians, mostly members of the security forces.
Colombia's high commissioner for peace, Luis Carlos Restrepo, said the tapes had been confiscated after the arrest of three suspected urban members of the Revolutionary Armed Forces of Colombia (Farc).
The three Americans were kidnapped four years ago
The video of Ingrid Betancourt carried the date of 24 October this year while those of the Americans were dated from 1 January.
Mr Restrepo said other evidence had also been seized, including some letters apparently written by the hostages.
One was from Mr Howes to his wife and another, dated 26 November 2006, was his will. Ms Betancourt had written a letter dated 24 October this year to her mother, Mr Restrepo said.
Leaders of the Farc, which holds hundreds of other hostages, have offered to free captives but only if a number of conditions are met, including being granted a demilitarised zone.
Mr Uribe opposes this, arguing that it would allow the rebels to regroup.
During recent mediation efforts by Venezuelan President Hugo Chavez, there were repeated calls for the guerrillas to produce "proof of life" but none was forthcoming.
Last week, Colombian President Alvaro Uribe ended Mr Chavez's role in trying to broker an exchange, accusing him of breaking protocol by being in direct contact with Colombia's army chief.
Mr Chavez's dismissal has sparked a diplomatic row between the two nations.
The Venezuelan leader on Wednesday vowed to have "no ties" with Colombia while Mr Uribe was in office.