"I place my resignation in your hands," wrote Tunisia ambassador to Unesco Mezri Haddad, in the letter to Tunisian President Zine El Abidine Ben Ali.
"I decided in soul and conscience to step down," he told AFP by telephone, saying he could no longer defend Tunisian state repression of demonstrators as an ambassador to the UN cultural body.
"I can no longer bear people being killed. I do not want anarchy in my country," he said.
In his letter, Haddad cited complaints he had made to Ben Ali about the police crackdown on demonstrations in which at least 66 people are said to have been killed in the past month.
The crisis in Tunisia continued Friday with police firing teargas at mass demonstrations in the capital Tunis against Ben Ali, who has ruled with an iron hand for 23 years.
In a bid to quell the unprecedented wave of political unrest, Ben Ali on Thursday promised in a national address that he would not seek another term in office in 2014 and vowed to liberalise Tunisia's political system and media.
He also said he had ordered police not to fire live rounds at demonstrators.
Haddad, a former political opponent of Ben Ali, said in the letter he had begged the president "to stop the bloodbath by disarming the police." "I told you that the protestors are not against you but against the oligarchy to which you have fallen hostage and which has plundered the country's riches without cease," he wrote.