FACTBOX - Who is Iraqi cleric Moqtada al-Sadr?

(Reuters) - Thousands of supporters of Shi'ite cleric Moqtada al-Sadr marched in Baghdad on Thursday to protest against a three-day-old crackdown against his followers and call for the end of the U.S.-backed government.

Here are some key details on Moqtada al-Sadr:

* Sadr, in his early 30s, led two uprisings against U.S. forces in 2004. The Pentagon once called the Mehdi Army the biggest threat to Iraq's security. U.S. officials and Sunni Arab leaders in the past have accused the militia of sectarian killings. Sadr has disavowed violence against fellow Iraqis.

* Sadr's movement ventured into national politics in 2005, and he was instrumental in appointing Nuri al-Maliki, a fellow Shi'ite, as prime minister in 2006. But Sadr pulled his movement out of the government in April 2007 when Maliki refused to set a timetable for the withdrawal of U.S. troops from Iraq. Sadr took his movement out of the ruling Shi'ite Alliance in September 2007. His group had held a quarter of the seats in the alliance.

* A fiery nationalist, Sadr has attracted a zealous following among the young and dispossessed. He derives much of his authority from his family. His father, highly respected Grand Ayatollah Mohammed Sadeq al-Sadr, was killed in 1999 for defying Saddam Hussein.

* Sadr rarely appeared in public in 2007 and U.S. officials say he spent much of the year in neighbouring Iran. Sadr's aides deny this. He has been taking advanced Islamic studies to earn credentials that would allow him to issue religious edicts.

-- Attaining higher religious credentials would enhance Sadr's influence among majority Shi'ites at a time when his movement is engaged in a bitter power struggle with another Shi'ite faction, the Supreme Islamic Iraqi Council, for influence in the oil-producing south. Some senior figures in the Shi'ite clerical establishment view Sadr as an upstart given his lack of scholarly achievement.

* Sadr ordered the Mehdi Army to freeze its activities for six months in late August after gunbattles among rival Shi'ite factions killed dozens of people in the holy city of Kerbala.

-- He undertook the move to weed out rogue elements of his militia and reassert his control. The U.S. military said the ceasefire helped bring violence down sharply in the latter part of 2007. He extended the ceasefire near the end of February for another six months.