BY TIM JOHNSON

 BOGOTA, Colombia -- In an angry challenge to Colombia's peace process, the
 leader of a right-wing paramilitary said Wednesday that his commandos seized
 and are holding the brother of a government peace negotiator.

 ``We are trying to impede the . . . progressive handover of the country to the
 guerrillas,'' outlaw leader Carlos Castaño said.

 Castaño said he will free the victim, Guillermo León Valencia, who was kidnapped
 in Medellín on Monday, once the government of President Andrés Pastrana offers
 a ``public report'' tallying the poor results of 20 months of peace talks with his
 archenemies, the leftist Revolutionary Armed Forces of Colombia (FARC).

 An aide to Pastrana reacted indignantly to Castaño's demands.

 ``It is an unacceptable way to pressure the negotiators,'' said Luis Guillermo
 Giraldo, a former opposition senator and member of a government team in talks
 with the guerrillas.

 Using harsh language in a statement to the private Radionet station, Castaño
 lashed out at the chief government negotiator, Fabio Valencia Cossio, elder
 brother of the kidnap victim, saying he had drifted too close to the guerrillas and
 tainted ``national dignity.''

 Valencia Cossio traveled to Spain earlier this month with a chief rebel
 spokesman, Raúl Reyes, for a political forum. Newscasts showed Valencia
 agreeing with Reyes on a number of issues, including a surprising affirmation that
 the rebels are not involved in drug trafficking.

 ``Now it turns out, according to Fabio Valencia, that they are the good guys and
 the rest of us Colombians are the bad ones,'' Castaño said. ``This is
 inadmissible.''

 Valencia, a former Senate president, is a stalwart of the governing Conservative
 Party and a likely future presidential candidate.

 Both leftist rebels and rightist insurgents in Colombia finance their campaigns by
 extorting money from rural landowners and protecting the coca crops that fuel the
 cocaine trade.

 2 KILLED IN ATTACK

 In a well-orchestrated kidnapping by a dozen or so commandos, Valencia's
 brother, one of 12 siblings, was snatched Monday at mid-day from his sport utility
 vehicle in a wealthy Medellín district. A policeman and an assailant were killed in
 the attack. The brother, 42, appeared to be taken unharmed.

 Castaño said rebel criticism of the kidnapping was hypocritical.

 ``Raúl Reyes accuses us of being enemies of peace because we carried out this
 retention, which he cynically condemns, even though the FARC carries out more
 than 1,000 kidnappings for ransom each year,'' Castaño said.

 Castaño, who leads the United Self-Defense Forces of Colombia, an outlaw army
 of some 7,000 to 11,000 combatants, said his group is not trying to force the
 ouster of Valencia as a peace negotiator.

 PUBLIC REPORT

 He said he wants Valencia to issue a public report on what has been gained
 during 20 months of government talks with the FARC, then compare it to ``the
 concessions made to the FARC so that all Colombians can draw their own
 conclusions.''

 In an effort to quell 36 years of insurgency, Pastrana in late 1998 offered the
 FARC a haven in south-central Colombia that is about a quarter the size of the
 state of Florida. The demilitarized zone is the site of on-again, off-again peace
 talks. Critics say rebels bring hundreds of kidnap victims and conduct military
 training in the zone.

 Polls show that average Colombians say they believe Pastrana has conceded too
 much to the guerrillas, although some experts in conflict resolution say the two
 sides have built up mutual trust critical to the success of future talks.