Voting in Friday's election kicked off at 7 a.m. (0230GMT) and is expected to continue until midnight (1930GMT), with the possibility of two hours' extension.
According to the Interior Ministry, around 59 million Iranians are eligible to vote this year, including 1.3 million first-time voters and 3.5 million Iranians voting abroad.
Despite passionate calls made by authorities for "maximum participation," turnout in major cities remained low until noon.
Among the first to vote was Iran's supreme leader, Ali Khamenei, who termed it a "day of the Iranian nation," urging people to cast their ballots.
Top government officials including President Hassan Rouhani, Vice President Es’haq Jahangiri, and parliament speaker Baqar Ghalibaf were also seen at different polling stations in the Iranian capital.
Jahangiri was one of the reformist candidates disqualified by the country's top election supervisory body from the race.
Some other disqualified candidates such as former speaker Ali Larijani, former IRGC commander Saeed Mohammad and Tehran City Council chief Mohsen Rafsanjani also cast their votes in Tehran.
Voting underway in Iran election, low turnout expected
Four candidates in the fray — judiciary chief Ebrahim Raeisi, former top banker AbdolNaser Hemmati, former IRGC chief Mohsen Rezaei and former deputy parliament speaker Ghazizadeh Hashemi —were also seen at different places, voting and urging people to vote.
There were unconfirmed reports that names of two candidates who dropped out of the race in favor of Raeisi — Saeed Jalili and Alireza Zakani — had not been removed from the ballot papers.
Some reports also pointed to disruptions at around 79 polling booths in the capital, and some reports suggested delay in voting in some places due to late delivery of equipment, which raised eyebrows.
Rouhani's reformist government is ending its two terms in office, with conservatives set to reclaim power, according to opinion polls.
The election, marred by disqualification of key reformist candidates, has set the stage for top conservative candidate Raeisi's landslide victory.