Candidates affiliated with Iran's Revolutionary Guards looked on course to win a parliamentary majority on Saturday, reportedly leading in the race in Tehran and towns and villages elsewhere.
An Interior Ministry official said a list of candidates affiliated with the Guards led in the capital.
Lists linked to hardliners captured 83 seats in towns and villages across the country following Friday's vote, according to a Reuters tally.
A clean sweep for hardliners would confirm the political demise of the country's pragmatist politicians, weakened by Washington's decision to quit a 2015 nuclear deal and reimpose sanctions in a move that stifled rapprochement with the West.
However, Iranian authorities have yet to announce the turnout in the race for the 290-seat legislature -- a litmus test of the popularity of hardliners closely associated with Supreme Leader Ayatollah Ali Khamenei.
Iran's rulers, under intense US pressure over the country's nuclear programme, need a high turnout to boost their legitimacy, damaged after nationwide protests in November.
Such a result would help the Guards, already omnipresent in Iranians' daily lives, to increase their substantial influence in political, social and economic affairs.
The demonstrations, which called for regime change, were met with a violent crackdown overseen by the Guards which killed hundreds and led to the arrest of thousands, according to human rights organisations.
Khamenei faces mounting pressure from the United States over Iran's nuclear programme and discontent over mismanagement of the economy is unlikely to ease as sanctions squeeze the Islamic Republic.
President Donald Trump raised the stakes in his standoff with Tehran when Iran's top general, Qassem Soleimani, was killed in a US drone strike at Baghdad airport in January.
Large gains in Friday's vote may also hand hardliners another bonus -- more leeway to campaign for the 2021 contest for president, a job with wide day-to-day control of government.
Parliamentary elections have little impact on Iran's foreign or nuclear policies, which are set by Khamenei, and major pro-reform parties have been either banned or dismantled since 2009.
But the vote reveals shifts in the factional balance of power in Iran's unique dual system of clerical and republican rule.
The Guardian Council, a hardline vetting body, has disqualified 6,850 hopefuls out of 14,000, ranging from moderates to conservatives, from contesting parliament polls. About a third of sitting lawmakers have also been barred.
Australian Associated Press