Iranians are voting an a presidential election, in which Hassan Rouhani is seeking a second term, the BBC reports.
Mr Rouhani, 68, a moderate cleric who negotiated a landmark nuclear deal with world powers in 2015, is standing against three other candidates.
His main challenger is seen as Ebrahim Raisi, 56, a hardline cleric and former prosecutor who is close to Supreme Leader Ayatollah Ali Khamenei.
If no-one wins more than 50% of votes cast, a run-off will be held next week.
Every incumbent president has been re-elected in Iran since 1985, when Ayatollah Khamenei himself won a second term.
He cast his ballot just minutes after polls opened at 8:00 local time (03:30 GMT).
“Everyone should vote in this important election,” he said, urging citizens to get the polls early.
Mr Rouhani voted about an hour later.
More than 54 million people are eligible to vote in Friday’s election.
Early election results are expected on Saturday.
