Iran is set to hold a runoff vote to elect a new leader on Friday, pitting a reformist who seeks dialogue with the West against a conservative hardliner.
The election will choose the successor to President Ebrahim Raisi, who died in a May helicopter crash.
No candidate gained a majority in last Friday's vote, triggering the runoff between the top two candidates.
The contest pits sole reformist candidate Masoud Pezeshkian, a former deputy parliamentary speaker and health minister, against hardliner Saeed Jalili.
Jalili previously served as secretary of the Supreme National Security Council, a body in charge of national defense and foreign affairs.
Under an international nuclear deal struck in 2015, Iran is supposed to restrict its nuclear program in exchange for an end to international sanctions against it.
But the United States pulled out of the agreement in 2018 under President Donald Trump and reimposed sanctions on Iran. Iran's economy has since been sluggish due to a plunge in the value of the rial and soaring prices.
Pezeshkian aims to reshape the nuclear deal and improve Iran's relations with Western nations.
Jalili is critical of the 2015 deal. He insists that even if Iran remains in conflict with the West, it can overcome sanctions by developing domestic industries and strengthening ties with emerging nations.
Figures published by the Iranian government show a 40 percent turnout for the June 28 election -- the lowest since the Islamic regime's establishment in 1979.
It's believed Pezeshkian will gain an advantage if voter turnout increases on Friday.
The election outcome is expected to be known as early as Saturday, Japan time.