Mr Colom says the video is part of a reaction to his crackdown on crime
President Alvaro Colom of Guatemala has vehemently denied playing any role in the murder of a prominent lawyer.
Rodrigo Rosenberg, who was shot dead on Sunday, had recorded a video saying Mr Colom and other officials were out to kill him and would be responsible.
"The video is totally false, my conscience is clear," Mr Colom said, and rejected calls for his resignation.
He told reporters that he was seeking help from the UN and the FBI to investigate Mr Rosenberg's killing.
The publication of the video has caused a outcry in Guatemala, correspondents say.
An editorial in the Prensa Libre newspaper said Mr Rosenberg's recording had "created the most serious political crisis for this democracy, because never before has a democratically elected president been accused of direct or indirect involvement in a murder".
Rodrigo Rosenberg's video has been shown repeatedly on Guatemalan TV
In the video, Mr Rosenberg is seen seated at a desk and speaking into a microphone.
Mr Rosenberg says: "If you are watching this message, it is because I was assassinated by President Alvaro Colom with help from Gustavo Alejos," a reference to the president's secretary.
The lawyer alleges that he was killed because of his links to a client, a prominent businessman, who was murdered in March.
Mr Rosenberg says his client, Khalil Musa, who was shot dead with his daughter, Marjorie Musa, was targeted because he was going to reveal government corruption.
President Colom went on national television on Monday flanked by his ministers to "categorically reject the accusations" and say a full and speedy investigation would be held.
And speaking at a news conference on Tuesday, Mr Colom again rejected the charges.
"I'm not a thug or a drug lord," Mr Colom said, adding that he had no idea what had motivated Mr Rosenberg to make the video. He suggested that both the murder and the video were a reaction to his attempts to tackle organised crime.
"The death of attorney Rosenberg has been used by political opportunists and traditional conspirators linked to organised crime to confuse public opinion and attack the top authorities," he said.
Mr Colom said he had asked for help from the FBI and also from a UN agency known as the International Commission against Impunity in Guatemala (CICIG). The commission was created in 2007 to tackle widespread corruption in the country.