The septuagenarian, who is considered to be the leading religious authority among Iraqi Sh'ias, was born in Mashhad, Iran, into a family of religious scholars.

He moved to Iraq at the age of 21 to pursue his religious studies under Grand Ayatollah Abd al-Qassim al-Khoi, and has remained there since. Al-Sistani resides in Al-Najaf, where his famous grandfather for whom he is named also studied.

Al-Sistani remained in Iraq during Saddam Hussein's rule, during which he was targeted for harassment on several occasions. He was jailed following the Shi'a rebellion after the first Gulf War in 1991, and his mosque was shut down in 1994.

He was appointed as al-Khoi's successor before his death in 1992. The Hussein regime's assassination of Grand Ayatollah Muhammad Sadiq al-Sadr in February 1999 cemented al-Sistani's status as the country's preeminent Shi'a authority.

During his absence from Al-Najaf after traveling to London on 6 August 2004 for treatment for a heart condition, the city became a center of clashes between U.S.-led forces and fighters loyal to rival cleric Muqtada al-Sadr.

Al-Sistani returned to the city on 25 August 2004 in a successful last-minute bid to negotiate an end to the three-week battle and spare the holy Imam Ali Mosque -- which was surrounded by U.S. forces and ensconced by Muqtada al-Sadr's Al-Mahdi Army -- from destruction.

Al-Sistani does not advocate violent resistance to the U.S.-led occupation of Iraq, but has shown no reluctance to defy U.S. plans when they conflict with his beliefs. For example, he issued a fatwa in June 2003 demanding the election, not appointment, of those writing the new Iraqi Constitution.

Al-Sistani currently heads the Imam Ali foundation.