A slain man's videotaped and posthumously broadcast accusation that President Alvaro Colom president ordered his murder threw Guatemala into an uproar and prompted government calls Tuesday for a U.N. agency and the FBI to investigate the killing.

Colom vehemently denied the allegations made in a videotape left by lawyer Rodrigo Rosenberg, who was shot to death by unidentified assailants while riding his bicycle Sunday. Opposition leaders and protesters called for Colom to step aside.

"If you are watching this message, it is because I was assassinated by President Alvaro Colom with help from Gustavo Alejos," the president's secretary, Rosenberg said in the video distributed at his funeral on Monday.

Rosenberg said on the tape that officials might want to kill him because he represented businessman Khalil Musa, who was slain in March. The lawyer alleged that Musa was killed because he refused to engage in acts of corruption that Colom purportedly invited him to participate in.

The Guatemala City newspaper Prensa Libre said the recording "has created the greatest political crisis for this democracy, because never before has a democratically elected president been accused of murder."

Television stations repeatedly broadcast the video and so many people watched it on Guatemalan Internet sites that some temporarily collapsed under the load.

Colom — center-leftist who took office in January pledging to fight poverty — went on national television to dismiss the accusations and demand an outside investigation.

He said his government has requested the International Commission against Impunity in Guatemala to investigate "to clear up this matter." The U.N. commission was created in 2007 to clean up corruption in Guatemala.

Colom said he also talked to U.S. ambassador Stephen McFarland to ask the FBI to probe Rosenberg's slaying. The U.S. embassy did not confirm this request nor said if the FBI would participate in the investigation.

Otto Perez, secretary-general of the leading opposition faction, the Patriot Party, called on Colom to step aside during the investigation.

More than 5,300 people joined a Facebook group called "Guatemalans for the dismissal of Alvaro Colom."