Protests against US and foreign forces in the country have been held every year on April 9, primarily by Sadr supporters, since Saddam Hussein, the former president, was ousted from power in 2003.
"Entering new era"
Sheikh Hazem al-Araji told Sadr supporters during Friday prayers that the country was "entering a new era, which has no place for occupiers and oppressors ... and people who put in jail those who resisted".
"Those who cling to their posts must not remain," he said, a comment apparently aimed at Nouri al-Maliki, the Iraqi prime minister.
Al-Maliki led a crackdown in 2008 on the Sadr movement's armed wing the Mahdi Armyand the group has made it clear they would not support him for another term in office.
In Baghdad, a protest organised by an "anti-occupation" group drew both Shia and Sunni Iraqis.
Demonstrators gathered in the predominantly Shia district of Kadhimiya, near the shrine of Imam Musa al-Kadhim. From there they planned to march east across al-Aaimmah bridge, which straddles the Tigris River, on to the predominantly Sunni neighbourhood of Adhamiya, organisers said.
The group said it seeks to have Shias and Sunnis unite in prayer and together call for an end to the presence of US and other foreign forces in the country.
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