Voting is also underway for city and village councils elections concurrent with the presidential polls. Voters are standing in long queues to cast their ballots in favor of their candidates.
Authorities appeared overwhelmed by the large number of people who began lining up even before polling stations opened.
Voting will last 10 hours and polls close at 6 pm, but voting hours could be extended if necessary. Ballot counting will start at 24:00 tonight and final results will be announced in the next 24 hours after the election.
Campaigning for today's presidential election kicked off in Iran after the Guardian Council released the list of 6 qualified candidates on April 21, although 4 have remained in the race now. Campaigning ended at 8:00 local time on May 18.
The qualified candidates represent a wide range of political streams from Principlists to Reformists.
There are now 4 candidates left in the race after two of them gave up their bid. They include incumbent President Hassan Rouhani, member of the Expediency Council Mostafa Aqa-Mirsalim, former deputy judiciary chief Seyed Ebrahim Rayeesi and former vice-president Seyed Mostafa Hashemi-Taba.
Tehran's mayor and a Principlist candidate Mohammad Baqer Qalibaf announced in a statement on Monday that he has withdrawn his presidential bid in favor of Rayeesi, another candidate of the same front who now has the highest chance among the Principlist nominees to win the election.
Also incumbent Vice-President Eshaq Jahangiri announced withdrawal in favor of President Rouhani on Tuesday afternoon.
The vetting body examined the qualifications of more than 1,600 candidates who had signed up to run in the election on May 19.
According to presidential election laws, all Iranian nationals above 18 years of age are eligible to vote.
Iranian voters will go to over 63,000 polling stations in and outside the country today to elect their next president.
Observers predict a high turnout similar to previous elections in the Islamic Republic in the post-revolution era.
The next president of the country will be elected if one of the four candidates manages to secure 50 percent plus one of the votes, otherwise, a runoff between the two top contenders will determine the next president.