Millions of Iranians voted to choose a new president on Friday, urged by Supreme Leader Ayatollah Ali Khamenei to turn out in force to discredit suggestions by arch foe the United States that the election would be unfair.

The country's 50 million eligible voters have a choice between six candidates to replace incumbent Mahmoud Ahmadinejad, but none is seen as challenging the Islamic Republic's 34-year-old system of clerical rule.

The first presidential poll since a disputed 2009 contest led to months of unrest is unlikely to change rocky ties between the West and the OPEC nation of 75 million, but it may bring a softening of the antagonistic style favoured by Ahmadinejad.

World powers in talks with Iran over its nuclear program are looking for any signs of a recalibration of its negotiating stance after eight years of intransigence under Ahmadinejad.

Voting in the capital Tehran, Khamenei called on Iranians to vote in large numbers and derided Western misgivings about the credibility of the vote.

"I recently heard that someone at the U.S. National Security Council said 'we do not accept this election in Iran'," he said.

"We don't give a damn," he added.

Three hours into Iran's presidential election, eye-witnesses said turnout appeared to be higher than expected in Tehran.

Long queues were seen in front of polling stations, especially in the southern and eastern neighborhoods of the capital. Observers had anticipated a low turnout following the fraud allegations and the violent suppression of post-vote protests that marred the 2009 poll.

Also, Iranian-Americans and Iranian expatriates in other nations will be able to vote Friday in Iran's presidential election, but turnout is expected to be lower than in 2009 in part because there are fewer places to cast ballots.

During the previous election, record numbers of Iranians voted in 41 locations throughout the U.S. This year, there are half as many voting locations.

On May 24, U.S. Secretary of State John Kerry questioned the credibility of the election, criticizing the disqualification of candidates and accusing Tehran of disrupting Internet access.

All the surviving contenders except current chief nuclear negotiator Saeed Jalili have criticised the conduct of diplomacy that has left Iran increasingly isolated and under painful economic sanctions.

Hossein, a 27-year-old voter in Tehran, said he would vote for the hardline Jalili, 47, Khamenei's national security adviser and a former Revolutionary Guard who lost a leg in the 1980-88 Iran-Iraq war.

"He is the only one I can trust to respect the values of the revolution ... He feels and cares for the needy," Hossein said.

In Zurich, a 40-year-old Iranian expatriate who gave his name as Manouchehr, said he would choose the most reformist-looking of the candidates, moderate cleric Hassan Rohani, since abstaining would not achieve anything.

The Guardian Council, a state body that vets all candidates, barred several hopefuls from the ballot, notably former president Akbar Hashemi Rafsanjani, one of the Islamic Republic's founding fathers seen as sympathetic to reform, as well as Ahmadinejad's close ally Esfandiar Rahim Mashaie.

This narrowing of the field prompted concerns of a low turnout which the supreme leader sought to counter.

"What is important is that everyone takes part," Khamenei said. "Our dear nation should come (to vote) with excitement and liveliness, and know that the destiny of the country is in their hands and the happiness of the country depends on them."

Iran's Sunni Muslim Gulf Arab neighbours are also wary of Shi'ite Iran's influence in neighbouring Iraq and its backing for President Bashar Assad and his Lebanese allies Hezbollah in the Syrian civil war. The Sunni Arab kingdoms are backing the rebels in Syria.

Inflexible stance

Of five conservative candidates professing unwavering obedience to Khamenei, only three are thought to stand any chance of winning the vote, or making it through to a second round run-off in a week's time.

Nuclear negotiator Jalili, who advocates maintaining a robust, ideologically-driven foreign policy, is seen as the main conservative contender.

The other two, Tehran mayor Mohammad Baqer Qalibaf and former foreign minister Ali Akbar Velayati, have pledged never to back away from pursuing Iran's nuclear programme but have strongly criticised Jalili's inflexible negotiating stance.

They face Rohani, the sole moderate candidate and only cleric in the race. Though very much an establishment figure, suspicious of the West, Rohani is more likely to pursue a conciliatory foreign policy.

With no independent, reliable opinion polls in Iran, it is hard to gauge the public mood, let alone the extent to which Khamenei and the Revolutionary Guards exert
their powerful influence over the ballot.

Voting lasts for 10 hours until 1.30 P.M. GMT, and the country's interior minister announced on Friday afternoon that voting would be extended. Around 1.6 million eligible electors are first-time voters aged 16 or more. While polling stations were scheduled to close in the afternoon, officials were expected to extend voting time until midnight local time.

2009 crackdown

Security has been tight and campaigning 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 Ahmadinejad was returned to office by results the reformists said were rigged.

The big protests that broke out were met by a crackdown in which several people were killed and hundreds arrested. The reformist candidates who lost in 2009 are now under house arrest and have little contact with the outside world.

Human rights groups have criticised Iran for further arrests and curbs on activists and journalists ahead of Friday's poll and the disqualification of 678 people registered as candidates.

Iranian officials dispute accusations of human rights abuses and call the charges politically motivated. They also say elections in Iran are free, fair and democratic.