Anti-U.S. cleric Moqtada al-Sadr presented himself on Thursday as a statesman who can unite a fractured Iraq, drawing tens of thousands of supporters to celebrate the departure of U.S. troops and flex his political muscle.

Two days after ministers from the main Sunni-backed political bloc eased a crisis with Shi'ite Prime Minister Nuri al-Maliki by returning to work, Sadr struck a markedly moderate note, calling on the fractious blocs to come together.

Sadr, a Shi'ite cleric whose Mehdi Army fought U.S. troops for years, remains an influential political player. It was not until he backed Maliki nine months after parliamentary elections in 2010 that Maliki was able to form a government.

If the last stage was of military and political resistance, the next stage will be that of construction and peace, Sadr said in a recorded speech broadcast to his massed supporters.

The crowd, which filled a main avenue in Baghdad's Sadr City neighbourhood, waved Iraqi flags and carried coffins representing those killed by American forces between the 2003 U.S.-led invasion and their withdrawal in mid-December.

Officially, the rally was a celebration of the withdrawal of U.S. forces after almost nine years, but it was also a reminder of Sadr's mobilising power as Maliki's government edges away from crisis.

Under a delicate power-sharing agreement, Iraq's ministries have been divided between Shi'ite, Sunni and Kurdish political blocs, but frequent disagreements between them have hampered progress on key items such as passing a 2012 budget.

Maliki moved against two top Sunni government members shortly after the U.S. troop withdrawal, prompting a walkout by lawmakers and ministers from the Sunni-backed Iraqiya bloc. They have since ended their boycott but the tensions remain.

RESISTANCE

Sunnis fear that Maliki is concentrating power in his hands at their expense, and the Iraqiya boycott was accompanied by a string of attacks on Shi'ite targets, prompting fears of a return to the sectarian bloodshed that peaked five years ago.

The head of Sadr's political bloc, which holds roughly an eighth of the seats in parliament, called in December for fresh parliamentary elections to end the bickering. A national conference is being planned, to try to reconcile all sides.

Sadr's message on Thursday contrasted sharply with the political deadlock. While his supporters carried banners with slogans such as The time of tyrants is gone, the time of building has come, he called for unity between faiths.

Our slogan was, will be and still is that Sunnis and Shi'ites are brothers and we won't sell this country, Sadr told the sprawling crowd, adopting a more moderate tone than his usual combative style.

Although Sadr and Maliki's parties are allies in the Shi'ite National Alliance bloc, Sadr has criticised Maliki over the recent political crisis, saying it could lead to one-party rule.

Sadr's position could also soon be undermined by the arrival on the political scene of former allies of his, such as Shi'ite militia group Asaib al-Haq, whose leader has said it is prepared to lay down its weapons and move into politics.

The stage at his rally was draped in the colours of the Arab Spring, representing a wave of popular uprisings that has swept from Tunisia across North Africa to the Middle East.

The flags of Tunisia, Libya, Egypt, Yemen and Bahrain hung behind the podium. Also on display were both the Syrian flag and another one used by opponents of President Bashar al-Assad, an ally of Shi'ite power Iran.

(Additional reporting by Suadad al-Salhy and Mohammed Ameer; writing by Francois Murphy; editing by Tim Pearce)