Iranians voted for 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 ballot, the first presidential poll since a disputed 2009 contest led to months of unrest, is unlikely to bring profound change in Iran's rocky ties with the West, but it might bring a softening of the antagonistic style adopted by outgoing President Mahmoud Ahmadinejad.

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

Iran's Gulf neighbours are also wary of its influence in Iraq next door and backing for President Bashar al-Assad and his Lebanese allies Hezbollah in the Syrian civil war. The Sunni kingdoms are backing the rebels in Syria.

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 US National Security Council said 'we do not accept this election in Iran'," he said. "We don't give a damn," he added.

On May 24 US Secretary of State John Kerry called into question the credibility of the election, criticising the disqualification of candidates and accusing Tehran of disrupting Internet access.

Iran's Guardian Council, the state body that vets all candidates, has barred a number of hopefuls from the roster in the ballot, including former president Akbar Hashemi Rafsanjani, who is seen as sympathetic to reform.

Iranians can choose from a slate of six candidates, all of whom were approved to run by the Guardian Council.

"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 deppends on them."

Of five conservative candidates professing unwavering obedience to Supreme Leader Ayatollah Ali 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.

Of those three main conservative hopefuls only one, current chief nuclear negotiator Saeed Jalili, advocates maintaining Iran's robust, ideologically-driven foreign policy.

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 a single moderate candidate, the only cleric in the race, Hassan Rohani. 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 will exert their powerful influence over the ballot.

State media reported that polling began on schedule at 0330 GMT. Voting lasts for 10 hours, though this can be extended if need be. There are more than 50 million Iranians eligible to vote, 1.6 million of them first-time voters.

But 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 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.-Reuters