In second place is the Conquest Alliance, made up of ex-fighters from mainly Iran-backed paramilitary units that battled IS.

In a system calibrated to divide parliament after the fall of dictator Saddam Hussein following the American-led invasion of 2003, the negotiations to form a governing coalition start a week after the end of the vote.

At the end of the May 12 vote, which saw record high abstentions with just 44.52 percent turnout, the unlikely Marching Towards Reform alliance between the populist Shiite preacher and Iraq's communists won 54 seats in the new Parliament.

The Conquest Alliance won 47 seats, ahead of the Victory Alliance, headed by Abadi, which had 42.

Sadr, who has ruled himself out of becoming prime minister, is looking to be the kingmaker and to cobble together a technocrat government from a dozen parties.

But despite leading the tally, his alliance falls short of a majority and it will take lengthy wrangling to forge a coalition.

Iran has already sent an envoy to Baghdad, the influential general Qassem Soleimani, who is rallying conservative Shiite forces to oppose any alliance with Moqtada Sadr.

The vote was a slap in the face to the widely reviled elite that has dominated Iraq since the 2003 US-led invasion that ousted Saddam Hussein.