With results counted in more than half of Iraq's 19 provinces, electoral officials said the second-largest number of votes went to a political alliance led by Hadi al-Amiri, a former militia leader with strong ties to Iran.
The political coalition of influential Shiite cleric Muqtada al-Sadr took an early lead in Iraq's national elections in partial returns announced late Sunday by the Iraqi electoral commission.
The announcement came just over 24 hours after polls closed across the country amid record low voter turnout.
During four years in office, Prime Minister al-Abadi boasted about many achievements, including the defeat of the Islamic State and preventing a Kurdish bid for independence that would have broken the country into separate parts.
How the vote tally translates into parliamentary seats will be announced later this week, Iraq's election commission said.
"We, in accordance with the Iraqi Constitution, will take full responsibility to lead the country, protect it, and defend its unity, interest, and sovereignty until the formation of the new government", Abadi stated.
A populist bloc organized by Sadr won the most seats in Iraq's parliamentary election, according to results from all 18 provinces released Monday.
Prime Minister Haider al-Abadi performed poorly across majority Shiite provinces that should have been his base of support.
Al-Abadi directed Iraqi forces to retake the city late a year ago after the Kurdish regional administration organized a referendum on independence that controversially included Kirkuk; federal forces moved in with little bloodshed as Kurdish forces withdrew.
A document being circulated among journalists and analysts by a candidate in Baghdad showed Sadr had won the nationwide popular vote with over 1.3 million votes, followed by Amiri with over 1.2 million and Abadi with over 1 million.
Sadr has reinvented himself as an anti-graft crusader after rising to prominence as a powerful militia chief whose group waged a bloody insurgency against USA forces after the 2003 invasion.
"He was not fearless enough to tackle the symbols of corruption, despite the major popular support", said journalist Kazem Ajrich. Authorities are seeking as much as $88 billion for postwar reconstruction.
The election came as the country deals with the disenfranchisement of the country's Sunni minority. His father, highly respected Grand Ayatollah Mohammed Sadeq al-Sadr, was killed in 1999 for defying Saddam Hussein.
If parliament chooses him as prime minister, Abadi will remain under pressure to maintain that balancing act amid tensions between Washington and Tehran over the nuclear accord.
"For the first time I can say congratulations to the leader and congratulations to the Iraqi people, congratulations on winning first place in Baghdad, and God willing we will be the first in Iraq", said Abbas Allawi, a candidate on the Sadr-backed Sairoon list.
Abadi, preferred by the US and credited for overseeing the crucial battle against Daesh as well as curtailing Kurdish independence ambitions, spearheads the Victory (Nasr) list. His allies campaigned on issues such as easing poverty, improving public services and combating corruption.