Polling stations opened at 8 a.m. local time (0430 GMT) on Friday and are scheduled to close at 6:00 p.m. (1430 GMT), with the possibility of extensions by Interior Ministry.
Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei cast his ballot at a polling station in Tehran just as the voting began.
Campaigning for the second round of voting started on Sunday, right after the Interior Ministry announced the results of the election held on June 28.
Candidates Masoud Pezeshkian and Saeed Jalili emerged as frontrunners, yet neither succeeded in obtaining an outright majority, necessitating the current runoff.
Pezeshkian has a background as a health minister and is a legislator from Tabriz in the northwest. Jalili holds experience as a diplomat and the chief nuclear negotiator.
More than 24 million people participated in the June 28 election.
The election seeks to appoint a successor to President Raisi, who passed away on May 19 in a tragic helicopter crash.
Mohsen Eslami, the spokesperson for the national election headquarters, indicated on Thursday that the number of polling stations remains consistent with the initial round, which stood at 58,640, although adjustments may be made in certain districts if needed.
He projected that preliminary results could be declared as soon as Saturday morning.
A total of 61 million individuals are eligible to vote, as confirmed by the election headquarters.
Press TV