"We must think about people's progress and hope of Iran and not to deceive the Iranian people", said President Rouhani. He will not confront the leader if elected.
The election is important "for Iran's future role in the region and the world", Rouhani, who struck a deal with world powers two years ago to curb Iran's nuclear program in return for the lifting of most economic sanctions, said after voting.
His opponent was the then president, Mahmoud Ahmadinejad, a firebrand hardliner who was looking to secure his second term in office.
The Guards hope that a win for Raisi will give them an opportunity to claw back economic and political power lost in Shi'ite Iran's complex theocratic and republican governing structure since 2015, when Iran struck a nuclear deal with world powers that brought it out of worldwide isolation. Rouhani enjoys Khamenei's blessing, otherwise he would not have been permitted to run.
Iranians overseas were also voting in over 300 locations, including 55 in the US, where more than 1 million Iranians live.
In addition, it is worth taking into account the charisma of these two political figures, who clearly overshadow the other presidential candidates, according to Kozhanov.
The Iranian Revolution of 1979 overthrew Washington's puppet ruler, the shah, and just with Castro's revolution in Cuba, the United States has never forgiven it for that crime, whereas by now Iranians have more or less forgiven the U.S. for the CIA-backed coup in 1953 that destroyed Iranian democracy and gave the shah supreme power in the first place.
The incumbent, relative moderate Hassan Rohani, and a conservative former prosecutor, Ebrahim Raisi, are widely regarded as the presidential front-runners in the clerically dominated Islamic country of around 81 million people, which is laboring under high unemployment and struggling to seize economic opportunities overseas. The government has consequently been unable to finance big infrastructure projects, and ordinary Iranians have seen few benefits.
Without mentioning his name, Rouhani criticized the policies of Ebrahim Raisi, who is the head of Astan-e Quds Razavi, Iran's largest foundation that is linked to the Imam Reza shrine in Mashhad, the holiest site in Iran for Shiite Muslims. Though he has been endorsed by the leaders of the 2009 Green Movement, Mr.Rouhani failed to deliver on his promise to have those leaders released from years of house arrest. "It comes after the country has resolved one of its major disputes with America (the nuclear issue), and my father told me that now there is no excuse for any candidate, pertaining to their good performance, in the next term", he said. However, his candidacy has revived the controversy surrounding the 1988 mass execution of thousands in Iran.
Both candidates urged voters to respect the outcome of the vote.
The centrists, led by the Rouhani, are hoping not to lose their current sovereignty over the conservatives.
However, regardless of how this struggle plays out behind closed doors and who is decided as the next president, ultimately all of the competing powerful factions will unite around the following objectives: to project the appearance of the election as having had a high turnout and level of enthusiasm among those eligible to vote, thus lending legitimacy to, and bolstering the mandate of, the eventual victor. A voter must show their national ID card and fill out a form.
Polls open at 0330 GMT and close at 1330 GMT, although authorities often extend voting hours into the evening.
Iranian elections are run by the country's interior ministry, which oversees the nation's police forces. And to ensure that the final results of the election look as if they have been exclusively determined by the people's ballot. Iran bars domestic and worldwide observers from the elections, bucking a widely accepted principle around the world that global watchdogs warn can allow for fraud.
"This is a polarised election - a race between powerful unelected centres of power and the rest of the country", said analyst Hamid Farahvashian. But real transformation in Iran will require a collapse of the Islamic Republic. It holds elections and has elected representatives passing laws and governing on behalf of its people. Despite what Khamenei says publicly about Rouhani, the supreme leader is satisfied with him.