DUBAI,  United Arab Emirates — Moderate cleric Hassan Rohani took a commanding lead over conservative rivals in Iran's presidential election, partial vote counts showed on Saturday, 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 - matters of national security that remain the domain of Supreme Leader Ayatollah Ali Khamenei.

But the president runs the economy and wields important influence in decision-making in the sprawling OPEC member state of 75 million people. Rohani could usher in a change from the confrontational style of President Mahmoud Ahmadinejad, who was constitutionally barred from seeking a third consecutive term.

Though an establishment figure, Rohani is a former chief nuclear negotiator known for his conciliatory approach. He has pledged to promote a foreign policy of "constructive interaction with the world" and to enact a "civil rights charter" at home.

Rohani's wide early margin revealed a broad reservoir of pro-reform sentiment with many voters, undaunted by restrictions on candidate choice and campaign rallies, seizing the chance to repudiate the dominant hardline elite over Iran's economic woes, international isolation and crackdowns on social freedoms.

In an apparent move 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 top Shiite cleric's official Twitter account said.

With some 27 million votes counted from the 50-million-strong electorate, Rohani had tallied 50.81 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 less than 16 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."

Small groups gathering outside Rohani's campaign headquarters in Tehran were politely asked to disperse by police, witnesses said, indicating authorities' desire to see no repeat of the crowds that gathered after the 2009 vote, but also more restraint on the part of security forces. The pro-Rohani groups moved on quietly, according to the witnesses.

Late on Friday, authorities estimated turnout would top 70 percent, a figure which is 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.

DECISIVE SPLIT

Rohani's campaign was endorsed by pragmatic former president Akbar Hashemi Rafsanjani after the latter was barred from running by a state vetting body out of concern, analysts said, that he could prove too potent a rival to Khamenei.

"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 to help consolidate the non-conservative vote.

In contrast, several high-profile conservatives with close ties to powerful clergy and Revolutionary Guards chiefs 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 Iran on Friday as millions turned out to cast their ballot in the first presidential race since the 2009 contest where allegations of fraud provoked 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 reject any accommodation with the West and reformers muzzled for the last four years who argue that the Islamic Republic needs to be more pragmatic in its relations with the world and modernize 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. (