Tens of thousands of Sadr supporters marched in Baghdad calling for an end to US presence in Iraq [Reuters]

Moqtada al Sadr, a prominent Iraqi Shia cleric, has threatened to revive his Mehdi Army and relaunch armed resistance against continued US presence in the country.

The threat came as tens of thousands of people marched across the capital Baghdad, marking the eighth anniversary of former leader Saddam Hussein's fall on Saturday.

A spokesperson of al Sadr, said the US had until the end of the year to meet the cleric's demands.

The Shia leader,  returned to Iraq from a self-imposed exile following a strong showing by his bloc in the 2010 parliamentary election in January 2011.

Al Jazeera correspondent Jane Arraf, reporting from Baghdad, said that this time Sadr had not only warned US troops but also the contractors.

"The rally marks the start of new campaign by one of the most powerful political forces in Iraq and it must be remembered that the Shia leader had fought against the US army in 2004," she said.

"The protests were also directed against the government for not providing jobs and basic services to the people."

Shortly after the US-led invasion of Iraq in 2003, the Shia leader had spoken out against the Coalition Provisional Authority, led by Paul Bremer.

He has continually criticised the US military presence in Iraq.

Al-Sadr froze his militia in 2007, dramatically reducing violence in the country.

His appeal to the poor and dispossessed accounts for much of his popularity, but some Iraqis support him as symbol of resistance against US presence.