FACTBOX: Five facts about Iraq's Moqtada al-Sadr

(Reuters) - Iraqi Shi'ite cleric Moqtada al-Sadr ordered his ministers to quit the government on Monday in protest at Prime Minister Nuri al-Maliki's refusal to set a timetable for a U.S. troop withdrawal.

Following are five facts about Sadr:

* Sadr's political group was instrumental in appointing Maliki, a fellow Shi'ite from the Dawa party, as prime minister. Sadr's group has a quarter of the seats in the ruling Shi'ite Alliance, parliament's largest bloc, and had controlled six ministries.

* Washington has called the Mehdi Army, a Shi'ite militia that claims loyalty to Sadr, the biggest threat to Iraq's security. U.S. officials and Sunni Arab leaders accuse the Mehdi Army of being behind many of the sectarian killings in Iraq. Sadr has publicly disavowed violence against fellow Iraqis.

* Sadr, whose stronghold is Baghdad's sprawling Sadr City, led two uprisings against U.S. forces in 2004. The former U.S. occupation authority issued an arrest warrant for him for his alleged role in the murder of a rival cleric soon after the invasion. The arrest was never carried out.

* Sadr, a fiery nationalist, derives much of his authority from his family. His father, highly respected Grand Ayatollah Mohammed Sadeq al-Sadr, was murdered in 1999 for defying Saddam Hussein. His father's uncle Mohammed Baqir, was killed by Saddam in 1980 after calling for an Iranian-style Islamic state.

* Sadr, in his early 30s, attracts a zealous following among the young, poor and dispossessed. His rising power has unsettled Shi'ite religious elders in the holy city of Najaf, many of whom see the junior cleric as a dangerous upstart. U.S. military officials say Sadr is in hiding in Iran to escape a security crackdown. His aides insist the cleric is in Iraq.