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Marwan, the Umayyad caliph was at that time advancing towards Kufa with a huge army. He encountered the army from Khorasan at a point on the greater Zab river, and the battle of Zab lasted for two days. It was closely contested struggle, and the day was turned when Marwan's horse ran away without its rider. He managed to escape, but was eventually discovered and killed.
So fell the last of the Umayyads in 132/750. The total duration of the Umayyad rule till the time when Abul Abbas assumed the power of the Abbasid rule was 90 years, 11 months and 13 days.