Polls opened Friday for Iran's runoff presidential election, the interior ministry said, pitting reformist candidate Masoud Pezeshkian against Saeed Jalili in the race to succeed Ebrahim Raisi, who died in a May helicopter crash.

Iran's supreme leader Ayatollah Ali Khamenei cast his ballot when the polls opened at 8am, Iranian TV showed.

"We are starting the second round of the 14th presidential election to choose the future president from among the two candidates across 58,638 polling stations in the country and all stations abroad," Interior Minister Ahmad Vahidi said, according to Iranian TV.

The vote comes against the backdrop of heightened regional tensions over the war in Gaza, Iran's dispute with the West over its nuclear programme and popular discontent at the state of the country's sanctions-hit economy.

In last week's first round, Pezeshkian, who was the only reformist permitted to stand, won the largest number of ballots, around 42%, while the former nuclear negotiator Jalili came in second place with 39%, according to figures from Iran's elections authority.

Only 40% of Iran's 61 million eligible voters cast their ballot — the lowest turnout in any presidential election since the 1979.

On Wednesday, Khamenei called for a higher turnout in the runoff.

"The second round of the presidential election is very important," he said in a video carried by Iranian TV.

He said participation was "not as expected" in the first round but that it was not an act "against the system."

Last week's vote saw the conservative parliament speaker Mohammad Bagher Ghalibaf come in third place with 13.8%, while cleric Mostafa Pourmohammadi garnered less than 1%.

Iran's presidential election was originally scheduled for 2025 but was brought forward by the death of Raisi in a May helicopter crash.