TEHERAN (AFP) - Moderate cleric Hassan Rowhani, bolstered by a late surge in support from suppressed Iranian reformists, was leading the race on Saturday to succeed Mahmoud Ahmadinejad as president, initial election results showed.

With 36.6 per cent of ballots counted, Mr Rowhani had 50 per cent of the vote, more than three times as many as his nearest rival, conservative Tehran mayor Mohammad Baqer Qalibaf on 15 per cent, said the interior ministry.

Under Iranian law, any candidate must clear a threshold of 50 per cent plus one vote to avoid having to face the second-placed candidate in a run-off, which would be held on June 21.

More than 50.5 million Iranians were eligible to vote to find a successor to Mr Ahmadinejad, who after serving two consecutive terms was constitutionally barred from standing for office again.

And voters reportedly turned out in massive numbers for Friday's election, with Mr Rowhani benefitting from the withdrawal this week of the only moderate candidate Mohammad Reza Aref.

Mr Aref, a former first vice president, pulled out of the race on Tuesday at the urging of former reformist president Mohammad Khatami, who then threw his weight behind the 64-year-old moderate cleric.

Mr Rowhani, a former top nuclear negotiator who has vowed to mend Iran's ties with the international community, garnered nearly 6,049,655 votes from 12,091,699 ballots counted by 12pm (0730 GMT), the interior ministry said on Saturday.

Mr Rowhani led talks with world powers over Iran's controversial nuclear ambitions under Mr Khatami's presidency.

During his tenure, in 2003, the Islamic republic agreed to suspend uranium enrichment. It was restarted after Mr Ahmadinejad first became president in 2005.

Iran has been at loggerheads with world powers over its nuclear ambitions, which the West suspects is aimed at developing atomic weapons. The stand-off has resulted in the imposition of harsh economic sanctions and Teheran's international isolation.

While campaigning, Mr Rowhani promised to move to ease those sanctions, which have led to severe economic pain in the country.

Inflation is raging at more than 30 per cent, the Iranian currency, the rial, has lost nearly 70 per cent of its value, and unemployment is rising.

On the votes counted so far, Mr Rowhani is followed by Mr Qalibaf with 15 per cent; ex-commander of the Revolutionary Guards Mohsen Rezai with 12.5 per cent; top nuclear negotiator Saeed Jalili with 11.5 per cent. All three hail from conservative camps.

Mr Jalili, Mr Qalibaf and ex-former foreign minister Ali Akbar Velayati, another candidate who is behind with only six per cent of the votes so far, are all considered close to supreme leader Ayatollah Ali Khamenei.

Mr Rowhani, who boasts of close relations with moderate ex-president Akbar Hashemi Rafsanjani, also touts his relation with Mr Khamenei, who has final decision on all key state issues, including the nuclear programme.

Mr Rowhani represented Mr Khamenei in the Supreme National Security Council, Iran' top security body, where he also served as its secretary for 16 years until 2005.

In the run-up to the disputed re-election of Mr Ahmadinejad in 2009, official election results were quickly released. But in this campaign, counting of the votes has proved to be time consuming.

Interior Minister Mostafa Mohammad Najjar said early on Saturday that his electoral staff would not "compromise accuracy for speed".

Ms Abbas Ali Kadkhodaei, the spokesman for the Guardians Council electoral watchdog, said late on Friday that "no violation" had been reported whatsoever across Iran.

Mr Ahmadinejad's re-election in 2009 sparked massive anti-regime street protests after his opponents and two reformist candidates, Mr Mir Hossein Mousavi and Mr Mehdi Karroubi, alleged wide-scale voting fraud.

Those protests, stifled by a brutal regime crackdown, led to the eventual detention under house arrest of Mr Mousavi and Mr Karroubi, and a widespread suppression of reformists.

Mr Rowhani's surge in the campaign came after Mr Khatami publically endorsed him, asking all his supporters to vote for the moderate candidate.

Mr Khatami's endorsement also gave birth to an online movement, with social networkers urging abstentionists not to waste their votes this time around.

"I will vote for Rowhani, even though I do not know him at all and did not want to vote until yesterday," said a man named Ali on his Facebook page.

"I will vote because the consensus of those wanting to save Iran is on Rowhani." Other net surfers have adopted Mr Rowhani's official colour - purple - and his symbol of a key to open the door to solutions for Iran's problems.