Polls will be open from 7:00 a.m. until midnight, local time.
As old and new U.S. sanctions hit Iran, trade dried up and foreign companies bolted.
The election victor will take over in August as Iran's eighth president from incumbent Hassan Rouhani, a moderate who has served the maximum of two consecutive four-year-terms allowed under the constitution. However, the state-linked Iranian Student Polling Agency has estimated a turnout of just 42%, which would be the lowest ever since the country's 1979 Islamic Revolution.
Social media posts from inside Iran also indicated that voting was light at polling stations around the country.
There are more than 59 million eligible voters in Iran, a nation home to over 80 million people.
Friday's election was dominated by Raisi, a protege of Iran's Supreme Leader Ayatollah Ali Khamenei, after the disqualification from the ballot of the competitors most likely to challenge him in the vote.
Iran's Guardian Council, the constitutional watchdog tasked with approving candidates, allowed only Raisi and six other lesser-known Khamenei loyalists to run in the election, barring hundreds of other presidential hopefuls, including several politicians who have prominent public profiles comparable to Raisi's. The disqualifications sparked a weekslong campaign by Khamenei's Iranian critics inside and outside the country to encourage a boycott of what they described as a sham election. "That is why the ruling system has always put emphasis on voter participation and used all possible means to bring people to the polls". "This stance is a big no to the Islamic republic", she said.
A win for Raisi would confirm the political demise of pragmatist politicians like Rouhani, weakened by the US decision to quit the nuclear deal and reimpose sanctions in a move that stifled rapprochement with the West.
State-linked opinion polling and analysts put hardline judiciary chief Ebrahim Raisi as the dominant front-runner in a field of just four candidates. Paper ballots, stuffed into large plastic boxes, were to be counted by hand through the night, and authorities said they expected to have initial results and turnout figures Saturday morning at the earliest.
The victor will take over in August as Iran's eighth president from incumbent Hassan Rouhani, a moderate who has served the maximum of two consecutive four-year-terms allowed under the constitution.
Iran has never acknowledged the mass executions, and Raisi himself has never publicly addressed allegations about his role.
Regardless of who wins, Khamenei remains Iran's ultimate decision-maker in matters of foreign and nuclear policy.
Whatever the ideological differences in Tehran, there is broad agreement that the country of 83 million must seek an end to the painful USA sanctions in talks aiming to revive the nuclear deal.
President Joe Biden's administration has said it could ease the sanctions if Iran agreed to resume curbs on nuclear activities that could be weaponized.
The incumbent president is not running, as Iran has a two-term limit for the presidency.
But high hopes for greater prosperity and a reopening to the world were crushed in 2018 when then-US president Donald Trump withdrew from the accord and launched an economic and diplomatic "maximum pressure" campaign against Iran.
Trump said the JCPOA did not do enough to prevent Iran from developing nuclear weapons - a goal it denies having - or engaging in other objectionable activities.
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