BEIRUT, July 4 (Reuters) - Lebanon's Grand Ayatollah Sayyed Mohammed Hussein Fadlallah, one of Shi'ite Islam's highest religious authorities, died on Sunday at the age of 74, a medical source at a Beirut hospital said.

Fadlallah had a wide following among Lebanese Shi'ites and was a supporter of Iran's Islamic Revolution. He was the spiritual leader and mentor of the Shi'ite militant group Hezbollah in the first years after it was formed in 1982.