According to partial official results obtained by AFP, the Marching Towards Reform alliance of Shiite cleric Moqtada Sadr and communists was ahead in six of Iraq's 18 provinces and second in four others.

The Conquest Alliance, made up of ex-fighters from mainly Iran-backed paramilitary units that battled the Islamic State group, was ahead in four provinces and second in eight others.

After a vote Saturday that saw a record number of abstentions, the Victory Alliance of Prime Minister Haider al-Abadi, who has been backed by the international community, looked to have won in only one province.

The complex electoral arithmetic of the Iraqi system means that the final makeup of the parliament is still far from decided.

Whatever the outcome, there looks set to be lengthy horse-trading between the main political forces before any new premier and government can be installed.

Abadi -- who came to power as IS swept across Iraq in 2014 -- is a consensus figure who has balanced off the United States and Iran.

The other leading challengers have often taken a stronger stance against the United States.

Several senior political figures had previously told AFP that preliminary results put Abadi in the lead, on course to scoop 60 of the 329 parliament seats up for grabs.

- Celebrations in Baghdad -

The ballot Saturday saw a record low turnout, as only 44.5 percent of eligible voters headed to the polls in the lowest participation rate since the 2003 US-led ouster of Saddam Hussein.

After the announcement that the Marching Towards Reform was ahead in Baghdad, supporters took the streets in the capital to celebrate a win.

Crowds of mainly young people waved flags and pictures of the populist nationalist cleric Sadr while fireworks fired off into the night sky.

Both Sadr and the leader of the ex-combattant Conquest Alliance, Hadi al-Ameri, have pitched themselves as looking to sweep clean Iraqi politics.

They are both long-time political veterans well known to Iraqis.

Sadr is also well known to Washington. In the years after 2003 invasion of Iraq, Sadr and the militia he controlled became a major thorn in the side of US military, waging a brutal and costly insurgency against coalition troops.

The low turnout and apparent strong showing for the more anti-establishment blocs showed that many Iraqis are fed up with the political elite that has dominated the country since the US-led invasion.

Across the war-scarred nation people railed against the same old faces who they accuse of corruption and sectarianism.

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.

Turnout was low despite a sharp decrease in violence across the country, with threats from IS against the polls failing to materialise.

US Secretary of State Mike Pompeo, whose country still has troops in Iraq from the fight with IS, on Saturday lauded the poll and called for an "inclusive government, responsive to the needs of all Iraqis".

Whoever emerges as premier will face the mammoth task of rebuilding a country left shattered by the battle against IS -- with donors already pledging $30 billion (25 billion euros).

Over two million people remain internally displaced across the country and IS -- while weakened -- still has the capability to launch deadly attacks.

The US-led coalition that helped battle IS pledged Sunday to work with the elected government to ensure the "lasting defeat" of IS and said the poll proved Iraqis "emphatically rejected violent extremism".