The left-wing insurgent group - formed in 1965 by intellectuals inspired by Fidel Castro and Che Guevara - is thought to have up to 5,000 members and a greater number of supporters.

The ELN is Colombia's second-largest leftist rebel group

It is small in comparison with Colombia's most powerful rebels - the 15,000-strong FARC, or Revolutionary Armed Forces of Colombia.

But the ELN nonetheless wages a war on the state and right-wing paramilitaries through hijacking, bombing, and extortion - often targeting foreign employees of large corporations, particularly those in the petroleum industry.

Along with the FARC, the ELN is fighting the military in Arauca, near the Venezuelan border, where it frequently assaults the energy infrastructure and has inflicted major damage on pipelines and the electricity distribution network.

Mistakes

But the fact that it is now releasing the last of seven foreign tourists it kidnapped in September should come as no surprise, say analysts.

Like other groups, they feel they must demonstrate flexibility if they are to win the public-relations war Professor Victor Bulmer-Thomas

Professor Victor Bulmer-Thomas, of London's Royal Institute of International Affairs, told BBC News Online: "The ELN is afraid of losing its profile.

"They have been eclipsed recently. But like other groups, they feel they must demonstrate flexibility if they are to win the public-relations war.

"This is an area where President [Alvaro] Uribe has been very successful - in persuading the Colombian people that they must back him in the struggle against rebel groups."

In February 2003, sources within the ELN admitted the group had made a mistake in kidnapping two foreign journalists, British reporter Ruth Morris and US photographer Scott Dalton.

The rebels admitted the move damaged their standing, particularly in Europe, where the group still has some support.

'Hard to negotiate'

In its early years, the movement attracted many Catholic priests who adhered to liberation theology, starting in 1966 with a handsome young crusading priest from a prominent family, Father Camilo Torres.

He was killed in his first action with the guerrilla group, but several other priests followed in his footsteps. Among them was Spaniard Manuel Perez, who led the movement from the 1970s until his death in 1998.

President Uribe has taken a tough stance on rebel groups

He was succeeded by the present leader, Nicolas Rodriguez, alias "Gabino", a tenacious guerrilla fighter and ladies' man who joined the movement as a teenager.

Unlike the FARC, who adopted a strict hierarchy and concentrated on building up their military power, the ELN members split their efforts between military and social work, adhering to their campaign for improved social justice and human rights.

The ELN did not move into the drugs trade in the same way as their more powerful cousins, in part due to the moral objections of Perez.

This is another explanation for their failure to enjoy the same explosive military growth as the FARC, but also - analysts say - for raising their profile as one of the more acceptable Colombian rebel groups.

"They have a long list of demands, but broadly campaign for better social justice, human rights and a more socialist system of economics," said Professor Bulmer-Thomas.

"But this makes them very difficult to negotiate with because the government can say that a democracy is in place in Colombia, it has limited resources and that it is doing what it can in these areas."

Churchgoers kidnapped

The ELN reached the height of its power in the late 1990s but in recent years has been hit hard by right-wing paramilitaries and the Colombian armed forces.

It hit the headlines with a series of mass kidnapping operations starting in April 1999, during which rebels hijacked a domestic airliner, forced it to land on a deserted jungle airstrip and kidnapped the passengers and crew.

This was swiftly followed by the abduction of an entire church congregation during a service in the city of Cali.

Guerrillas burst into the church and herded 150 worshippers and the priest into waiting trucks.

Colombia has a highly stratified society, separating the traditionally rich families of Spanish descent from the vast majority of poor Colombians, most of whom are of mixed race.

With few avenues for social mobility, this has provided a natural constituency for left-wing insurgents like the ELN.

800 kidnappings

The Colombian Government broke off peace talks with the ELN in mid-2002, saying the guerrillas were not committed to peace.

Figures from 2001 showed that the ELN carried out more than 800 of about 3,000 kidnappings in Colombia that year.

But the group had been showing a will to make peace since June 1998, when rebel leaders met with Colombian civilian and church representatives in Germany.

Observers of the peace process say, however, that the group had been ignored by the government in favour of the larger FARC.

President Uribe's tough stand against rebel groups has affected hopes of a settlement with the insurgents.