Saylor.org's Ancient Civilizations of the World/From Arab to Islamic Empire: The Early Abbasid Era
Establishment of the Abbasid Caliphate 
In 747, the Abbasids rose in rebellion against the Umayyads in order to capture the throne and realize their age-old claim to the leadership of the Arab world by virtue of their being descendants of Muhammad's yongest uncle Abbas. They received the ardent support of the Persians, Jews, Christians and Egyptians. The Persians participation in the rebellion , in particular, was enormous and had it not been for the overwhelming support of the Persians which the Abbasids received they would not have been able to capture the throne.
In 750, matters became so serious that the Umayyad Caliph Marwan II was forced to take note. He led a large Umayyad force into Iraq to strike the rebelling Abbasids. But despite the disparity of numbers, the Umayyad forces were overwhelmed at the Battle of the Zab on January 25, 750. Marwan fled to Egypt where he was captured and killed. Abu al-`Abbās `Abdu'llāh as-Saffāh ibn Muhammad, an Abbasid, became the Caliph of the Islamic world establishing the Abbasid dynasty.
"The Establishment of the Abbasid Caliphate" (Wikibooks) http://en.wikibooks.org/wiki/Iranian_History/The_Abbasids#Establishment_of_the_Abbasid_Caliphate