Moderate Iranian cleric Hassan Rohani looked to be heading towards an outright victory over his conservative rivals in the presidential election, initial results showed on Saturday.

The vote is unlikely to radically alter ties between Tehran and the West, but if Rohani wins he has indicated he would pursue a less confrontational foreign policy than current President Mahmoud Ahmadinejad and enact a "civil rights charter" at home.

With more than 5.2 million votes counted from the 50 million electorate, Rohani had 52.30 per cent of votes cast, Iran's interior ministry said. That takes Rohani above the 50 per cent of the vote he needs to avoid a second round run-off on June 21.

Under the election rules, a candidate has to win more than 50 percent of the total votes cast to win outright. A first round winner gaining less than that must compete with the runner-up in a second round a week later.

Rohani's nearest rival was conservative Tehran Mayor Mohammad Baqer Qalibaf, a long way behind with 17.33 per cent. Other hardline candidates scored even lower.

Rohani received significant boosts earlier this week when reformist candidate, Mohammad Reza Aref, withdrew in his favour. His campaign was also endorsed by former presidents Mohammad Khatami and Akbar Hashemi Rafsanjani.

In contrast, Iran's big-hitting conservatives failed to organise themselves around a single candidate, suffering what appeared a decisive split in their support base as a result.

Voting was extended by several hours at polling stations across the country on Friday as millions of Iranians turned out to cast their ballot in the first presidential race since a disputed 2009 contest led to months of political unrest.

Press TV reported that turnout was about 80 per cent.

Security has been tight and campaigns subdued compared to the euphoric rallies that preceded the last presidential election in 2009, when reformist supporters thought they scented victory and the prospect of change in Iran.

Those hopes were dashed when rapid announcements gave Ahmadinejad 63 percent of the vote, returning him to office and starting a series of deadly protests that lasted for months.-Reuters