BOGOTA -- Human rights defenders in Colombia are under constant attack for their work, facing murder, death threats, illegal surveillance, arbitrary detentions and prosecutions, activists told a congressional panel in Washington on Tuesday.

Speaking before the House Human Rights Commission, Colombian activist Gabriel Gonzalez recounted how he spent more than a year in jail on charges of being a member of the country's leftist guerrillas. A judge threw out the charges as baseless, but the ruling was overturned and he could face another seven years in prison on the same accusation.

His is one of dozens of cases, U.S. and Colombian rights groups say, where human rights defenders are prosecuted based on flimsy charges as part of an effort to intimidate them.

Margaret Sekaggya, United Nations Special Rapporteur on the situation of human rights defenders, who visited Colombia in September, told the panel that she remained concerned over what she has called a ``pattern of harassment and persecution against human rights defenders.''

Sekaggya challenged the government of President Alvaro Uribe to ``genuinely address'' their concerns.

Rights activists and community organizers have long been among the primary targets of both right-wing paramilitary forces and leftist rebel armies in Colombia, with more than 60 murdered between 2002 and 2008. Violence has abated greatly with the demobilization of more than 30,000 paramilitary fighters and the routing of guerrillas from major urban areas.

But last year, 11 rights activists were murdered, according to the Colombian Commission of Jurists, and in the first nine months of this year, nine rights defenders have been reported killed.

On Saturday, activist Islena Rey narrowly escaped death when members of the Revolutionary Armed Forces of Colombia, or FARC, reportedly opened fire on the small boat she was traveling in after holding meetings with far-flung communities in Meta province.

And while killings are down, ``more insidious forms of persecution have emerged,'' says Andrew Hudson of the New York based Human Rights First.

Investigations have shown that rights defenders are routinely subjected to surveillance and their phone calls and e-mails are illegally intercepted. The headquarters of rights groups are frequently the target of mysterious burglaries where only computers and memory sticks are stolen. And activists are prosecuted based on often flimsy charges.