After Imam Reza (A.S.) martyrdom, Mamun wrote a letter to people of Baghdad and the Abbasids, that since both Fazl bin Sahl and Imam Reza (A.S.), had been remove from the scene, there was no reason for their hostility against him. He, however, did not receive a favourable response, and becoming worried about the situation, decided to head towards Baghdad. From Sanabad he started for Gorgan where he stayed for a month to tighten security arrangement for Khorasan in his absence. From Gorgan he headed day city of Rayy (presently a southern suburb of modern day Tehran), where he stayed a few days before marching for Nahrvwan in Iraq.

At Nahrwan Abbas and the high ranking military officials of Baghdad .On 16th Safar, in 204 AH (819 AD), he entered Baghdad to overthrow Ibrahim bin Mahdi who had assumed the title of caliph for the past year and seven months.

Upon his entry into Baghdad, Mamun ordered the state colour to be changed black of the Bni Abbas, from the green of the Bani Hasem which he had made after Imam Reza's (A.S.) nomination in order to mislead the masses.