Abdurrahman Shalgam, an ally of Gaddafi since the pair were teenage radicals in the late 1950s, compared the leader's actions to those of Pol Pot and Hitler and backed the protesters in Tripoli.
In an emotional speech to the UN Security Council in New York, Mr Shalgam, who had previously remained loyal, said: "Muammar Gaddafi is telling the Libyans 'either I rule you or I kill you'." He told the 15 members of the council, who are considering an Anglo-French plan for sanctions against the Gaddafi regime: "We need a courageous resolution from you".
Outside the chamber, he gave another speech in which he pleaded for the outside world to do something "within hours, not days" to stop the bloodshed in the country.
Mr Shalgam said Gaddafi had lost the support of "90 per cent" of his diplomats and predicted further revolution in the middle-east.
"Slavery and the rule of one person is finished – it's finished," he said