Sen. Piedad Cordoba was sitting in the waiting room of a Medellin clinic, leafing through her appointment book, when more than a dozen masked and heavily armed assailants burst through the door.

Cordoba, president of the Senate human rights commission, was blindfolded and whisked into a waiting car. Then she was flown by helicopter to a mountain hideout where she met the country's most powerful right-wing paramilitary leader, Carlos Castano, whose forces she had accused of committing atrocities as part of their long conflict with Marxist rebels.

In a room dimly lit by a candle, Castano "accused me of being an ally and a spokesperson for the guerrillas," Cordoba recalled in an interview. "He showed me printed transcripts of my telephone conversations and insisted that I was collaborating" with one of the rebel groups.

Cordoba was released two weeks after her May 21 kidnapping, but not before Castano used the high-profile abduction to reaffirm publicly his intolerance for human rights advocates, whom he has accused of working "at the service of guerrilla diplomacy."

As Colombia's 36-year-old civil war has entered a heightened phase of attack and counterattack over the past two years, human rights defenders have been the victims of a campaign of terror, largely at the hands of illegal right-wing militia groups, many of which have ties to local units of the country's armed forces.

Since 1997, at least 27 human rights advocates have been slain in Colombia, five of them this year. The dead include workers at nongovernmental organizations, professors, lawyers, union members and municipal employees in charge of receiving complaints about human rights abuses. The vast majority of the killings have occurred in the northeastern state of Antioquia, where paramilitary groups have a strong presence.

Human rights organizations have been closing their offices in the interior of the country, some moving their operations to the capital, and so far this year more than 20 activists have sought political asylum in other countries. Recently, the head of the human rights unit of the Attorney General's Office was transferred after he received threats on his life.

"Unfortunately, it is true that those who work for peace and human rights in Colombia are targeted by certain sides making war and literally have to run for their lives," said Daniel Garcia-Pena, director of the office of the high commissioner for peace from 1995 to 1998 under former president Ernesto Samper.

Garcia-Pena, who said he relocated to the Washington area last October after receiving two threatening letters from Castano, added: "The terrible truth is that human rights workers have been forgotten in Colombia and are only remembered when they are killed."

The crisis confronting human rights activists has impeded their work at a time when Colombia's rural population is being subjected to an increasing number of atrocities and other war-related hardships. In the first half of this year, 860 people were killed in 186 separate attacks, while tens of thousands were displaced as a result of the conflict.

"The human rights movement in Colombia has been weakened, and the future does not look very good," said Anders Kompass, director of the Colombian office of the U.N. High Commissioner for Human Rights. "We are finding ourselves more and more alone. A lot of the [human rights] leaders have left the country, and those who are here are scared."

But right-wing paramilitary groups, whose estimated 5,000 combatants are largely financed by wealthy landowners, are not the only ones targeting human rights workers. Investigators say the military has collaborated with right-wing death squads in a number of attacks.

Despite a lack of cooperation by the armed forces, sources said federal investigators are probing whether a top army official worked with Castano to arrange the May 1997 killings of two human right workers who were shot while sleeping in their Bogota apartment.

On the other side of Colombia's conflict, the Marxist rebels have likewise been tied to violence against rights workers. In March, the Revolutionary Armed Forces of Colombia, the nation's largest rebel force, seized and killed three Americans working for indigenous rights in circumstances that remain unclear.

"The country is so polarized as a result of this nightmarish internal conflict, and the issue of human rights is so politicized, that there is a tendency to overlook atrocities committed by the forces one may sympathize with," said Jose Miguel Vivanco, executive director of the Americas division of Human Rights Watch.

Said Manuel Granado, a Bogota-based member of Peace Brigades International, whose unarmed volunteers accompany activists as a deterrent to violence: "It is too easy to kill a human rights worker in Colombia. There is little political cost. This is a situation we are trying to change."

Activists have complained that despite allocating more than $4 million to fund a protection program for social activist groups, the administration of President Andres Pastrana, like previous governments, has fallen short on its promises to provide such necessities as armed guards, security cameras and bulletproof vests. Others say that the program itself is insufficient.

Vice President Gustavo Bell, whose office has assumed the responsibilities of the defunct Colombian post of high commissioner for human rights, was unavailable for comment. But Pastrana, who has made peace the top priority of his presidency, says the government is doing its best to protect human rights advocates while facing an escalating civil conflict and an economic crisis.

But, said one human rights worker who changed residences here because of threats: "No matter what, you feel very helpless when you pick up the phone and hear machine-gun fire on the other end." Other activists have received fliers announcing their own funerals.

Indeed, trying to improve the human rights situation here has long been a perilous undertaking. The Attorney General's Office has about 80 open cases -- some dating back a decade -- that involve the killings or disappearances of a broad range of human rights proponents.

"In some areas of the country, it is suicidal to work on human rights, particularly in those areas full of paramilitaries" said Human Rights Watch's Vivanco. Castano, for instance, has declared Colombia's human rights community "a military target."

Observers said the large number of unsolved cases underscores the difficulty of apprehending the militia leaders allegedly behind the killings, men who have amassed weapons arsenals and generally seek refuge in remote mountain or jungle outposts, at times enjoying the protection of the military.

Furthermore, investigators are strapped by chronic shortages of resources that hamper inquiries and result in case backlogs. Pedro Diaz, the new coordinator of the attorney general's human rights staff, said the office "was designed to function in an era of normalcy, which means a war of low intensity, not a war that is out of control." Of the 25 prosecutors who started in the unit when it was established in 1994, all but five have left, largely because of threats and intimidation.

Prosecutors also have been frustrated in their efforts to investigate military personnel they suspect of involvement in slayings, because the army command routinely seeks to have the cases placed under the jurisdiction of a military tribunal that is believed never to have convicted a high-ranking officer of a human rights violation.

In such a climate, said Juan Manuel Bustillo, director of an umbrella association of 13 nongovernmental organizations: "Threats on many occasions are not reported. Unfortunately, we do not have confidence in the authorities."