The run-off follows a June 28 ballot with a historic low turnout, when more than 60 per cent of Iranian voters abstained from the snap election for a successor to Ebrahim Raisi following his death in a helicopter crash.
The low participation is seen by critics as a vote of no confidence in the Islamic Republic.
Hardline former nuclear negotiator Saeed Jalili hopes to become Iran's next president. (AP PHOTO)
Friday's run-off vote will be a tight race between low-key lawmaker Masoud Pezeshkian, the sole moderate in the original field of four candidates, and hardline former nuclear negotiator Saeed Jalili.
Polls open at 8am local time (0430 GMT) and close at 6pm (1430 GMT), but are usually extended until as late as midnight.
The final result will be announced on Saturday, although initial figures could come out sooner.
While the poll will have little impact on the Islamic Republic's policies, the president will be closely involved in selecting the successor to Ayatollah Ali Khamenei, Iran's 85-year-old supreme leader who calls all the shots on top state matters.
Khamenei acknowledged on Wednesday "a lower than expected turnout", but said, "it is wrong to assume those who abstained in the first round are opposed to the Islamic rule".
Voter turnout has plunged in the past four years, which critics say shows support for the system has eroded amid growing public discontent over economic hardship and curbs on political and social freedoms.
Ayatollah Ali Khamenei says Iranians who don't vote are not necessarily opposed to Islamic rule. (AP PHOTO)
Only 48 per cent of voters participated in the 2021 election that brought Raisi to power, and turnout was 41 per cent in a parliamentary election in March.
The election coincides with escalating regional tension due to the war between Israel and Iranian allies Hamas in Gaza and Hezbollah in Lebanon, as well as increased Western pressure on Iran over its fast-advancing nuclear program.
The next president is not expected to produce any major policy shift on Iran's nuclear program or change in support for militia groups across the Middle East, but he runs the government day-to-day and can influence the tone of Iran's foreign and domestic policy.
The rivals are establishment men loyal to Iran's theocratic rule, but analysts said a win by anti-Westerner Jalili would signal a potentially even more antagonistic domestic and foreign policy.
A triumph by Pezeshkian might promote a pragmatic foreign policy, ease tensions over now-stalled negotiations with major powers to revive the nuclear pact, and improve prospects for social liberalisation and political pluralism.
Both candidates have vowed to revive the flagging economy, beset by mismanagement, state corruption and sanctions reimposed since 2018 after the US ditched Tehran's 2015 nuclear pact with six world powers.
The hashtag #ElectionCircus has been widely posted on social media platform X, with some activists at home and abroad calling for an election boycott, arguing that a high turnout would legitimise the Islamic Republic.
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