The new president will succeed Mahmoud Ahmadinejad, the controversial leader who in his eight years in power saw Iran engulfed by economic turmoil and opting for a collision course with the West over nuclear programme.
The polling started at 8 am local time and will continue till 6 pm. Some 50 million people are casting their votes in the election. If no candidate wins 50.1% of the votes polled, a run-off will be held on June 21. No foreign observer was monitoring the poll.
Rougani has attracted much attention in the run-up to the polls, speaking publicly about Tehran's need to re-engage with the West. He has also promised to free political prisoners and called for media reforms.
Support for Rouhani increased after Mohammed Reza Aref, who was the only reformist candidate in the race, declared his withdrawal from the presidential race on the advice of ex-President Mohammed Khatami, who was a pro-reform one. Rouhani has been endorsed by Khatami and another former President, Akbar Hashemi Rafsanjani. The latter was disqualified from the race by the powerful Guardian Council.
The hardliner candidates in the election include top nuclear negotiator Saeed Jalili and Tehran's Mayor Baqer Qalibaf. The rest of the candidates are conservatives close to Iran's supreme leader Ayatollah Khameini. The leader, after casting his vote, told on the state television that the people of Iran will create a new political epic. He also slammed the USA for criticisng the presidential poll.
Friday's election is the first in Iran since 2009. The results that year were alleged to be rigged in favour of Ahmadinejad and serious protest had spread across Iran. However, the disqualification of Rafsanjani in May left the post-2009 liberal movement supporters divided on whether to vote at all.