FACTBOX: Key facts about Iraqi Shi'ite cleric Sistani

(Reuters) - Iraqi Shi'ite cleric Moqtada al-Sadr will disband his Mehdi Army militia if Grand Ayatollah Ali al-Sistani and senior Shi'ite clergy based in Iran order him to do so, a senior aide to Sadr told Reuters on Monday.

Here are key facts about Sistani, Iraq's most senior Shi'ite cleric:

* Sistani was born on Aug 4, 1930 in the northeastern Iranian city of Mashhad. He migrated to the holy Iraqi city of Najaf in his early 20s where he studied Shi'ite theology.

* He succeeded Grand Ayatollah Abdul-Aala al-Sibziwari in 1993 to become the most widely followed Shi'ite cleric in Iraq and some other predominantly Shi'ite areas in the region.

* The revered Sistani has been the leading voice of religious moderation in postwar Iraq, where he wields great influence.

* The white-bearded cleric steered clear of politics during Saddam Hussein's time but emerged as one of the most powerful men in postwar Iraq. He keeps away from the public eye and has been a recluse at his home in Najaf for years, only leaving briefly for heart treatment in Britain in 2004. He has refused to meet Western officials.

* Sistani is viewed by many as a moderate. He brokered a deal with Sadr to end an uprising by the Mehdi Army against U.S. and Iraqi forces in Najaf in August 2004. Sistani has repeatedly urged Shi'ites not to retaliate to attacks by Sunni militant groups such as al Qaeda. In June 2007, he urged followers to protect all shrines and mosques after Shi'ites in Baghdad and Basra attacked several Sunni mosques in retaliation for the destruction of two minarets at a Shi'ite shrine in Samarra.

* Iraq's ruling Shi'ite Alliance is a coalition of Shi'ite Islamist parties formed in 2004 with the blessing of Sistani. It has dominated Iraqi politics since the country's first post-war elections in January 2005. Politicians regularly visit the ageing cleric for advice on disputes and public support. Sistani once criticized politicians for spending too much time abroad.