BOGOTA, Colombia (AP) _ Heavily armed gunmen disguised as federal agents burst into a medical clinic Friday and kidnapped an opposition senator active in Colombia's fledgling peace process.

Piedad Cordoba was snatched from the lobby of an orthopedist's office in the upscale El Poblado neighborhood of Medellin, the country's second largest city, 155 miles northwest of Bogota, said presidential anti-kidnapping czar Jose Alfredo Escobar.

Eleven men carrying assault rifles and dressed in black overalls like the ones used by federal prosecutor's agents intercepted Cordoba in the clinic's lobby at midday. Three other men and a woman seated in the lobby in civilian clothes drew guns and helped overpower the senator's two government bodyguards.

They told Cordoba she would be freed with a message about the peace process, said Escobar, citing eyewitness accounts.

Right-wing paramilitary gunmen or leftist rebels from the National Liberation Army, known as the ELN, could be responsible, said state security agency director Col. German Jaramillo.

The ELN hijacked an Avianca airplane last month and abducted all 46 passengers and crew. It has been demanding more attention from the government in the peace process, which has focused on talks with the far larger rebel group, the Revolutionary Armed Forces of Colombia, or FARC.

President Andres Pastrana called on the kidnappers to free Cordoba, 44, a Liberal Party member on the Senate's human rights committee.

Cordoba has been active in promoting a negotiated settlement to the country's 35-year civil war, meeting frequently with armed factions.

On Thursday, she was a panelist at a conference on the peace process in the troubled northern state of Bolivar, where the ELN forced down the aircraft and the site of frequent clashes between guerrillas and their paramilitary rivals.

Colombia has the highest kidnapping rate in the world, with more than 1,500 cases reported every year.