BAGHDAD (Reuters) - Moqtada al-Sadr has been called champion of the poor, kingmaker and killer. Now the young Shiite cleric has emerged as a lightning rod in postwar Iraq's deepest crisis.

Police accused elements of Sadr's black-clad Mehdi Army militia of attacking Sunni mosques and homes after the bombing of a Shiite shrine on Wednesday sparked sectarian bloodshed and prompted Iraqi leaders fearful of civil war to appeal for calm.

Though Sadr and his aides have angrily denied ordering any violence, the quick appearance of Sadr's fighters in central Baghdad after the blast was a reminder that he is a force to be reckoned with as Iraqi leaders struggle to ease sectarian strife and form a new government more than two months after elections.

The combination of conciliatory rhetoric and paramilitary muscle may further enhance his influence in talks on forming a government which he has already transformed by throwing his weight behind Ibrahim al-Jaafari as prime minister, thwarting the hopes of a rival Shi'ite faction.

About 20,000 of Sadr's followers called for Shi'ite and Sunni unity during Friday prayer's and the young cleric told his followers in a statement not to attack Sunnis or their mosques.

But police said his fighters clashed with gunmen in another part of Baghdad, leaving Iraqi security forces who were trying to enforce a curfew helpless to stop them.

In the southern city of Basra, Sadr's representatives whipped up emotions by blaming the shrine attack on the Shi'ite -led interim government, from which Sadr has kept his distance.

"This happened with the blessing of the elected government, God's curse upon it, and because the government did not care because it follows what America wants and nothing else is important," said Karim al-Ghizzi, at Sadr's office in Basra, where thousands gathered for Friday prayers.

The crisis has pushed the unpredictable Sadr back to centre stage after he led two revolts against U.S. troops in 2004 and then manoeuvred his way into the biggest bloc, the Alliance, in the new parliament, earning a reputation as a kingmaker.

Sadr derives much of his authority from his highly respected father, a cleric believed killed by Saddam Hussein's agents.

He has gained popularity by continuing to provide services to the poor, creating a vast network of support not unlike that enjoyed by the Palestinian group Hamas, now in government.

Though U.S. forces smashed the Mehdi uprisings two years ago, dealing with Sadr could be a sensitive task for the Iraqi government if the crisis escalates.

Sadr's tour this month, taking in Iran and Arab states, suggested he was seeking a wider role across sectarian boundaries. Yet contradictions abound, with Mehdi militiamen -- or at least fighters dressed like them -- occupying Sunni mosques and hanging black Shi'ite flags from their minarets.

After he won Sunni Arab respect with his uprisings against U.S. occupation troops, such attacks on Sunni mosques prompted the main Sunni bloc to launch fierce criticism of Sadr.

"We all respected Sadr when he was fighting the Americans but now his Mehdi Army has strayed into activities that harm Sunnis and Iraqi unity," said Sunni politician Hussein Shukr.

But Sadr, whose popularity belies his relative youth and appears to irk more senior clerics, has survived graver charges.

An arrest warrant issued for him over alleged involvement in the murder of a rival cleric in Najaf in 2003 has never been acted on by the authorities, however. Sadr denies any role in the killing of Abdul Majid al-Khoei, who was hacked to death in the Imam Ali shrine, the holiest Shi'ite site in Iraq.