From staff and wire reports

 BOGOTA, Colombia -- The government of President Andres Pastrana has withdrawn from participation in a rebel-hosted international conference on production of drug crops in Colombia after rebels allegedly detonated an explosive "necklace" Monday, killing 53-year-old dairy farmer Elvia Cortez and a policeman.

 Pastrana, speaking at a police ceremony in Bogota, said, "We're horrified, we're indignant, but most of all we're determined to end this barbarity."

 Military officials said the killing was a bungled extortion attempt, but a rebel spokesman denied that the Revolutionary Armed Forces of Colombia, or FARC, was responsible for the attack in central Boyaca province. Colombia's chief prosecutor said Tuesday that his investigation had just begun.

 The deaths sparked outrage and calls from the Roman Catholic Church bishop and senior justice officials for a halt in peace talks.

 "The men of violence have placed a necklace of dynamite ... around the hope of all Colombians," Pastrana said in his toughly worded speech at a Bogota police academy.

 Cortez and police bomb disposal expert Jairo Lopez died, and three soldiers were maimed, when the "necklace bomb" -- possibly detonated by remote control -- exploded after nearly nine hours of efforts to defuse it. Lopez lost his left arm and right hand in the blast and bled to death as he was flown to a hospital by helicopter.

 Armed forces chief Gen. Fernando Tapias said FARC rebels rigged the device -- a collar of PVC pipe and dynamite -- around the woman's neck to force her husband to pay $7,500, which the guerrillas traditionally refer to as a "war tax."

 But guerrilla spokesman Commander Ivan Rios on Tuesday said no FARC fighters were even in the vicinity when the crime was committed. "We had nothing to do with this condemnable act," Rios told Radionet Radio.

 The conference on illicit drug crops is to start May 29 in the huge rebel-controlled southern enclave from which Pastrana pulled his troops as a gesture of peace. Ambassadors and delegates from 21 countries, including the United States, Canada and Europe, have been invited along with U.N. special adviser Jan Egeland.

 The delegates are to inspect projects to wean peasants from planting the coca and opium crops that are helping to fuel Colombia's 36-year civil conflict.

 Pastrana also condemned a FARC attack on a passenger bus in central Huila province last week in which six civilians died.

 "The FARC's attitude must change," Pastrana said. "Forty million Colombians who want peace are demanding that change. I demand this (rebel movement) demonstrate ... its will for peace."

 FARC attacks and kidnappings have continued despite peace talks begun last year, and Pastrana has been under public pressure to take a harder line against the rebels.

                           The Associated Press and Reuters contributed to this report.