Factbox: What is Iran's Assembly of Experts?

(Reuters) - Senior clerics in Iran's Assembly of Experts start a two-day meeting on Tuesday expected to focus on post-election unrest and Western pressure over Tehran's nuclear program. The meeting is also likely to test the influence of its chairman, former President Akbar Hashemi Rafsanjani.

Here are some facts about the Assembly of Experts:

* The 86-member assembly, founded in 1982, is a clerical body that supervises, appoints and in theory can sack the Supreme Leader -- an untested prerogative that hardliners contest. It has rarely intervened directly in policy-making. Its mandate is also to interpret the constitution.

* Direct elections for its members are held every eight years and are next due in 2014. As in presidential and parliamentary elections, the Council of Guardians determines who can run. In the last election for the assembly in October 2006, the Council of Guardians barred 300 candidates, ruling out many potential opposition members and all the women hopefuls.

* Rafsanjani, a centrist who favors economic liberalization and better ties with the West, was picked in September 2007 to lead the assembly, in a setback to conservative rivals and hardliners close to President Mahmoud Ahmadinejad.

* The assembly is based in the holy city of Qom, but also holds sessions in Tehran and the eastern city of Mashhad.

(Writing by David Cutler, London Editorial Reference Unit)