Polls had initially been slated to close at 6 p.m., but were extended until 8 p.m. to accommodate 'a rush of voters.'
More than 60,000 polling stations have been operating around the country to serve Iran's 56 million eligible voters. Reports say the polls have seen a strong turnout thus far in an unexpectedly tight race.
Iranian men queue to vote for the presidential and municipal councils elections, in the city of Qom, south of the capital Tehran, Iran, May 19, 2017.
President Hassan Rouhani could face a strong challenge from conservatives who have blasted his handling of Iran's economy and the nuclear deal he signed with the West.
Rouhani banked his political future on the landmark nuclear deal, which led to the loosening of economic sanctions on Iran. But Rouhani's main challenger, Ebrahim Raisi, has criticized the deal because it left in place sanctions on banking and financial services.
FILE - Iranian President Hassan Rouhani waves to his supporters in a campaign rally for the May 19 presidential election in Tehran, Iran, May 9, 2017.
Rouhani is seen as the favorite by political analysts and in the small amount of polling data that's been produced. He also has history on his side, as all incumbent presidents have won re-election since 1981.
But if Rouhani fails to secure a majority of the vote Friday among the four candidates, the decision would head to a runoff a week later between the top two vote-getters.
Conservative hard-liner Raisi has run on a largely economic platform, promising to create millions of jobs and fix the country's 12.7 percent unemployment rate. Raisi is seen by many as close to Ayatollah Ali Khamenei, Iran's current supreme leader, though Khamenei has not gone so far as to endorse Raisi.
FILE- Iranian cleric Ebrahim Raisi registers his candidacy for the May 19 presidential elections at the Interior Ministry in Tehran, Iran, April 14, 2017. More than 1,600 people registered to run.
Under Iran's system of government, which was implemented after the 1979 Islamic Revolution, all candidates must be vetted by a religious panel prior to gaining approval to run for president. While the supreme leader has ultimate say over Iran's state policies, the president still holds considerable influence in government.
Pro-reform candidate Mostafa Hashemitaba, who previously ran for president in 2001, and former culture minister Mostafa Mirsalim are the other two candidates in the race.