Presidential candidate Hassan Rohani casts his ballot during the Iranian presidential election in Tehran
Moderate cleric Hassan Rohani took a solid lead over conservative rivals on Saturday in preliminary vote counting in Iran's presidential election in what could be the makings of a surprise victory over favored hardliners.
The outcome is unlikely to transform relations between Iran and the outside world, the Islamic Republic's disputed policy on developing nuclear power or its support of Syria's president in the civil war there - all sensitive security matters that are the domain of Supreme Leader Ayatollah Ali Khamenei.
But the president does wield important influence in decision-making in the sprawling Shi'ite Muslim nation and major OPEC state of 75 million and could bring a change from the confrontational style of President Mahmoud Ahmadinejad, who was constitutionally barred from seeking a third consecutive term.
Rohani's wide early margin revealed a major reservoir of pro-reform sentiment whereby many voters seized a chance to repudiate the dominant hardline elite over Iran's economic woes, international isolation and crackdowns on personal freedoms despite restrictions on candidate choice and campaigning.
If he wins, Rohani, a moderate who is a former chief nuclear negotiator known for his conciliatory approach, has signaled he will promote a foreign policy based on "constructive interaction with the world" and enact a "civil rights charter" at home.
In an apparent attempt to convey political continuity to both domestic opponents and Western adversaries, Khamenei said that whatever the result of Friday's election, it would be a vote of confidence in the 34-year-old Islamic Republic.
"A vote for any of these candidates is a vote for the Islamic Republic and a vote of confidence in the system," the hardline clerical leader's official Twitter account said.
With some 23 million votes counted from the 50-million-strong electorate, Rohani had tallied 51.07 percent of all ballots cast, Iran's interior minister said. That would be enough to avoid a second-round run-off on June 21.
Rohani's nearest rival was conservative Tehran Mayor Mohammad Baqer Qalibaf, a long way behind with 16.3 percent. Other hardline candidates close to Khamenei, including current nuclear negotiator Saeed Jalili, scored even lower.
British former Foreign Secretary Jack Straw, who dealt with Rohani during nuclear negotiations between 2003 and 2005, called him a "very experienced diplomat and politician".
"This is a remarkable and welcome result so far and I'm keeping my fingers crossed that there will be no jiggery-pokery with the final result," Straw told Reuters, alluding to accusations of widespread rigging in the 2009 election.
"What this huge vote of confidence in Doctor Rohani appears to show is a hunger by the Iranian people to break away from the arid and self-defeating approach of the past and for more constructive relations with the West," he said.
"On a personal level I found him warm and engaging. He is a strong Iranian patriot and he was tough, but fair to deal with and always on top of his brief."
Suzanne Maloney, senior fellow at the Brookings Institution, said Iran "appears to be on the verge of shocking the world".
"With Rohani leading the vote, the regime's calculation now is whether a run-off campaign ... is worth the risk. A second round would entail an additional week of the kind of exhilarated campaigning, replete with young Iranians dancing in the streets and an amplified chorus of demands for social and political reforms, and ultimately pose a greater risk to the system."
Excitement was rippling through Rohani's campaign headquarters with workers there preparing for victory, said a source close to the campaign. The Rohani campaign expected an announcement in the coming hours, the source said.
Electoral officials did not say from which districts the votes so far counted had come from. Late on Friday, authorities estimated turnout would top 70 percent - relatively high and likely to benefit Rohani.
Iran's rial strengthened about 4 percent against the U.S. dollar on Saturday, web sites which track the currency said.
Rohani's campaign was endorsed by centrist former president Akbar Hashemi Rafsanjani after the latter, a veteran rival of Khamenei, was barred from running by a state vetting body.
"Iran has held the most democratic elections in the world and there are no uncertainties about it," Rafsanjani was quoted by Fars news agency as saying on Saturday.
Rohani received another big lift when reformists led by ex-president Mohammad Khatami swung behind him after their own lackluster candidate Mohammad Reza Aref withdrew in his favor.
In contrast, several high-profile conservatives with close ties to the ruling clerical or Revolutionary Guards elite failed to unite behind a single candidate, suffering what appeared to be 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 turned out to cast their ballot in the first presidential race since the 2009 contest where allegations of fraud led to mass unrest.
Rohani came to prominence as Iran's nuclear negotiator in talks with Britain, France and Germany between 2003 and 2005 that Tehran Iran agree to suspend uranium enrichment-related activities, easing Western pressure on Tehran.
He left the post when Ahmadinejad came to office in 2005. Enrichment work resumed and there has been virtually no progress in intermittent talks since then. The result has been a punishing expansion of international sanctions against Tehran, seriously damaging its heavily oil-dependent economy.
Rohani would be an important bridge between hardliners around Khamenei who oppose any accommodation with the West and reformers sidelined for the last four years who argue the Islamic Republic needs to be more pragmatic in its relations with the outside world and change at home in order to survive.
Security was tight during the election and campaigning subdued compared to the euphoric rallies that preceded the last presidential vote in 2009, when reformist backers thought they scented victory and the prospect of democratization.
Those hopes were dashed when rapid announcements awarded Ahmadinejad 63 percent of the vote, returning him to office and unleashing a tide of protests that lasted for months and led to dozens of killings and hundreds of arrests.