* Supreme Leader's loyalists get over 75 pct of parliament seats
* Ahmadinejad may be lame duck for rest of term
* Election held under sanctions pressure in nuclear row (Adds Khamenei extending influence, Khamenei ally, run-off votes)
By Parisa Hafezi and Hashem Kalantari
TEHRAN, March 4 (Reuters) - Clerical Supreme Leader Ayatollah Ali Khamenei has tightened his grip on Iran's faction-ridden politics after loyalists won over 75 percent of seats in parliamentary elections at the expense of President Mahmoud Ahmadinejad, a near-complete count showed.
The widespread defeat of Ahmadinejad supporters - including his sister, Parvin Ahmadinejad - is expected to reduce the president to a lame duck after he sowed divisions by challenging the utmost authority of Khamenei in the governing hierarchy.
The outcome of Friday's vote, essentially a contest between conservative hardline factions with reformist leaders under house arrest, will have no big impact on Iranian foreign policy, notably its nuclear stand-off with the West. But it will boost Khamenei's influence in next year's presidential election.
With 90 percent of ballot boxes counted, Khamenei acolytes were expected to occupy more than three-quarters of the 290 seats in the Majlis (parliament), according to a list published by the interior ministry on Sunday.
In the race for the 30 seats in the capital Tehran, a Reuters tally of preliminary returns showed Khamenei supporters had taken 19 and pro-Ahmadinejad candidates the rest. Leading in popularity was Gholam-Ali Haddad Adel, a key ally of Khamenei and father-in-law to the paramount leader's son, Mojtaba.
Pro-Khamenei candidates won in the Shi'ite Muslim holy cities of Qom and Mashhad and led in other major provincial centres including Isfahan and Tabriz, where over 90 percent of voters backed Ahmadinejad in the 2009 parliamentary election.
Khamenei loyalists also appear to have swept up around 70 percent of seats in rural regions - hitherto bastions of Ahmadinejad and his populist nationalism that clerics see as a threat to their political primacy in the Islamic Republic.
Independents and women candidates fared relatively well in many provincial towns, where they campaigned on the immediate concerns - generally economic -- of their constituents.
Iran's energy-driven economy is suffering badly from Western sanctions - now expanding to block its lucrative oil exports - imposed over its refusal to halt sensitive nuclear activity and give unfettered access to U.N. nuclear inspectors.
The interior ministry said final results were expected by Monday but the exact makeup of the new parliament will not be known until April, following runoff elections for more than thirty seats.
The results are hard to compare with the outgoing parliament since Khamenei and Ahmadinejad loyalists were united in the 2008 legislative elections, garnering about 70 percent of seats.
But analysts said the combative Ahmadinejad - who is constitutionally barred from running for a third presidential term - would not readily bow to the ballot box rout of his supporters and may fight back.
"Ahmadinejad's camp has not been demolished. We have to wait and see what happens after the new parliament convenes in June," said analyst Hamid Farahvashian.
"The vote showed that there is a deepening rift between the ruling elites. It might emerge in the coming weeks."
Ahmadinejad is likely to be summoned to an unprecedented hearing in the outgoing parliament by Friday to answer questions focusing on his rocky handling of the economy, while Khamenei kept ultimate control over foreign policy.
Critics say Ahmadinejad has inflicted higher inflation on Iranians by slashing food and fuel subsidies to cut spending and purge waste, and replacing them with cash handouts of around $38 a month per person.
Parliament could impeach Ahmadinejad if his explanations are unconvincing, but Khamenei's green light would be needed.
Analysts said Ahmadinejad is likely to survive his term - but as a lame duck president.
"The establishment is under Western pressure and does not want to look divided," said analyst Babak Sadeghi. "Ahmadinejad will finish his term as a weak executive."
Under mounting Western pressure over its nuclear programme and concerns that Israel might attack, Iran's clerical elite needed a high election turnout to shore up their legitimacy damaged since Ahmadinejad's 2009 re-election, in which fraud allegations triggered eight months of anti-government protests.
Khamenei, 72, said a high turnout would be a message of defiance to "the arrogant powers bullying us", a reference to Western states and sanctions against Iran.
State officials said the turnout was over 64 percent, higher than the 57 percent in the 2008 parliamentary vote.
Absent from the vote were the two main opposition leaders. Mirhossein Mousavi and Mehdi Karoubi, who ran for president in 2009, have been under house arrest for more than a year.
Iran denies Western suspicions that it is enriching uranium with the ultimate goal of developing nuclear weapons, saying the programme is for peaceful energy only.
But arch-adversary Israel has talked of war if diplomacy and sanctions do not bring about a peaceful outcome to the nuclear row. Iran will top the agenda when Israel's prime minister meets U.S. President Barack Obama in Washington on Monday.
Obama has said military action was among the options to prevent Iran acquiring nuclear weapons but has also argued against a pre-emptive Israeli strike.
Global oil prices have spiked to 10-month highs on tensions between the West and Iran, OPEC's second biggest crude producer. (Additional reporting by Mitra Amiri, Hossein Jaseb and Ramin Mostafavi; Writing by Parisa Hafezi and Marcus George; Editing by Mark Heinrich)