Supporters of anti-US Shiite cleric Moqtada al-Sadr, whose militia fought pitched battles with American forces, on Thursday officially celebrated the departure of the "occupiers" from Iraq.

The last American soldiers except for a small number under US embassy authority departed Iraq in mid-December, after almost nine years in the country.

But the official Sadrist celebration was held on Thursday after the end of Arbaeen, the 40-day period of mourning following the Ashura commemorations, which mark the death in battle of Imam Hussein, a formative event in Shiite Islam.

Tens of thousands of people turned out for the event, which was held in Sadr City in northern Baghdad, an area named for Moqtada's father, Grand Ayatollah Mohammed Sadiq al-Sadr, who was killed along with two of Moqtada's brothers in 1999 by gunmen allegedly sent by dictator Saddam Hussein.

"The armies of resistance terrified the occupiers, so they left after they lost," Moqtada said in a recorded message broadcast on a large screen at the celebrations.

American forces "turned from being a liberating army, as they said, into an occupying army," he said.

"The occupying forces were working for strife and destruction and to destabilise security. The occupier is not the one who can bring peace and safety to Iraq, but rather you, and only you."

At the urging of the cleric, his supporters shouted, "Yes, yes, to unity, yes, yes, to peace, yes, yes, to resistance."

Sadr also called "on the government to release the resisters," in an apparent reference to insurgents detained by Iraqi authorities.

Thousands of Sadr Movement members marched in formation with Iraqi flags at the event, while supporters gathered on the roadside, some holding banners reading, "No, no to America, no, no to Israel."

Among those attending the event, which was held under tight security, were cabinet ministers, members of parliament and religious figures, as well as representatives of some Arab countries.

Hazem al-Araji, one of the leaders of the Sadr Movement, told AFP that, "Today is the day of the real victory for the people and a message of unity that we throw in the face of the occupier."

"The occupier can never divide the Iraqis," Araji said.

"Today, the Sadr Movement is sending a positive message to all of the Iraqi people requesting that they preserve the country's unity," Abbas al-Bayati, an MP from Iraqi Prime Minister Nuri al-Maliki's State of Law list told AFP.

Sheikh Mohammed Abdullah, a Sunni, travelled from Anbar province to join the celebrations.

"We are participating today because there is no difference between Sunnis and Shiites, and everyone called for resisting the occupier," he said.

Sadr's Mahdi Army militia, which the cleric has since deactivated, fought -- and lost -- brutal battles with US forces in Najaf in February and August 2004, and with Iraqi forces backed by the Americans in Sadr City in March 2008.