By Huda Majeed and Ahmed Aboulenein
BAGHDAD (Reuters) - Moqtada al-Sadr was leading in Iraq's parliamentary election with over half of the votes counted, the electoral commission said on Sunday, pointing to a surprise comeback for the powerful Shi'ite cleric who had been sidelined by Iran-backed rivals.
Shi'ite militia leader Hadi al-Amiri's bloc, which is backed by Tehran, was in second place, according to the count of over 95 percent of the votes cast in 10 of Iraq's 18 provinces.
Prime Minister Haider al-Abadi appeared to be running third. Security and commission sources had earlier said he was leading the election, which was held on Saturday and is the first since the defeat of Islamic State militants inside the country.
Turnout was 44.52 percent with 92 percent of votes counted, the Independent High Electoral Commission said - that was significantly lower than in previous elections. Full results are due to be officially announced on Monday.
Sadr and Amiri both came in first in four of the 10 provinces where votes were counted, but the cleric's bloc won significantly more votes in the capital Baghdad, which has the highest number of seats.
The commission did not announce how many seats each bloc had gained and said it would do so on Monday after announcing the results from the remaining provinces.
Abadi, a rare ally of both the United States and Iran, came in third in six provinces but ran fifth in Baghdad.
The results unexpectedly showed former Prime Minister Nuri al-Maliki, who was touted as a serious challenger to Abadi, lagging behind.
The ranking of these blocs can still change with results yet to be announced from eight provinces, including Nineveh, which has the second-largest number of seats after Baghdad.
Abadi was viewed as a frontrunner before the election. His rivals were seen as Maliki and Amiri, both closer than Abadi to Iran, which has wide sway in Iraq as the primary Shi'ite power in the region.
A Sadr victory or second-place finish would mark a surprise comeback by the cleric, who has a zealous following among the young, poor and dispossessed but has been sidelined by influential Iranian-backed figures such as Amiri. Sadr has kept Tehran at a distance.
Sadr has formed an unlikely alliance with communists and other independent secular supporters who joined protests he organized in 2016 to press the government to see through a move to stem endemic corruption.
He derives much of his authority from his family. Sadr's father, highly respected Grand Ayatollah Mohammed Sadeq al-Sadr, was murdered in 1999 for defying Saddam Hussein. His father’s cousin, Mohammed Baqir, was killed by Saddam in 1980.
Whoever wins the election will have to contend with the fallout from U.S. President Donald Trump's decision to quit Iran's nuclear deal, a move Iraqis fear could turn their country into a theater of conflict between Washington and Tehran.
Abadi, a British-educated engineer who came to power four years ago after Islamic State seized a third of Iraq's territory, received U.S. military support for Iraq's army to defeat the Sunni Muslim militant group even as he gave free rein to Iran to back Shi'ite militias fighting on the same side.
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.
Abadi was seen by some Iraqis as lacking charisma and ineffective. He had no powerful political machine of his own when he took office.
But the defeat of Islamic State and Abadi's campaign to eradicate Iraq's rampant corruption improved his standing.
Amiri's Badr organization played a key role in the battle against Islamic State. But some Iraqis resent his close ties to Tehran. The dissident-turned-militia leader spent more than two decades fighting Saddam from exile in Iran.
Whoever wins the most seats still must negotiate a coalition government, which must be formed within 90 days of the election.
(Reporting by Huda Majeed and Ahmed Aboulenein; Writing by Michael Georgy; Editing by Catherine Evans and Paul Simao)