Shiite militia leader Hadi Al Ameri's Conquest Alliance, made up of ex-fighters from mainly Iran-backed paramilitary units that battled ISIS, was in second place after winning four provinces and placing second in eight others.
Iraqi Prime Minister and head of the al-Nasr Coalition Haider al-Abadi has expressed his willingness to work and cooperate in forming a stable government free of corruption.
Sadr is a Shia candidate, who is said to have benefitted from the low voter turnout of 44.5 percent.
Two Iraqi anti-establishment alliances appeared set to create an upset Monday as partial results showed them leading in a parliamentary election, ahead of the incumbent prime minister's bloc.
The results included Baghdad, where the Sadr-backed list won significantly more seats than Abadi's.
The final official results were expected today, after which a long process of allotting seats in the legislature will begin, followed by members electing a president and a prime minister.
In the years following the US -led invasion of Iraq in 2003, the young cleric Muqtada al-Sadr burst onto the scene as a renegade champion of poorer Shiites, leading militant fighters who carried out deadly attacks on American forces and were notorious for sectarian killings against Sunni Muslims.
Abadi encouraged all political parties and electoral lists that have concerns over the polling process to use legal procedures to contest the preliminary elections results.
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Abadi was widely seen as the front-runner.
In 2016 al-Sadr remerged as a central figure in Iraq's political scene by taking a lead role in the anti-government protest movement that bubbled up amid a deepening economic crisis as the war against IS dragged on.
He derives much of his authority from his family.
His father's cousin, Mohammed Baqir, was killed by Saddam in 1980.Whoever wins the election will have to contend with the fallout from US.
Celebrations erupted on the streets of Baghdad after the commission's announcement, with thousands of Sadr's supporters singing, chanting, dancing and setting off fireworks in the capital while carrying his picture and waving Iraqi flags.
Iraq has been ranked among the worlds most corrupt countries, with high unemployment, rife poverty, weak public institutions and bad services despite high oil revenues for many years.
The vote came with tensions surging between the United States and Iran after Washington's withdrawal from the 2015 nuclear deal, sparking fears of a destabilising power struggle over Iraq. Within the years after 2003 invasion of Iraq, Sadr and the militia he managed grew to become a serious thorn within the facet of United States army, waging a brutal and expensive insurgency towards coalition troops.