Ever since the martyrdom of Ali, fourth caliph among the Khulafa Rashidun and the son-in-law of Muhammad, in 661, which led to the establishment of the Umayyad Caliphate, a small part of the Muslim community rejected the Umayyads as usurpers and called for the establishment of a regime led by a member of the Ahl al-Bayt, the Family of Muhammad. The Abbasids, who claimed descent from Muhammad's paternal uncle Abbas ibn Abd al-Muttalib, profited from this during their rise to power against the Umayyads; but their claim too was rejected by the Shi'a, who insisted on the exclusive right of the descendants of Husayn and Hasan, Ali's sons by Muhammad's daughter Fatimah. A line of Imams emerged from the offspring of Husayn, who did not openly lay claim to the caliphate, but were considered by their followers as the true representatives of God on earth.