Early results from Iran’s presidential election put the reformist-backed candidate, Hassan Rouhani, in the lead.
Official figures give him just over half the 16 million votes counted so far – well ahead of second-placed Tehran mayor Mohammad Baqer Qalibaf.
Mr Rouhani could win in the first round if he finishes with more than 50%.
Electoral officials said turnout was high among the 50 million Iranians eligible to vote on Friday for a successor to Mahmoud Ahmadinejad.
His eight years in power have been characterised by economic turmoil and Western sanctions against Iran over its nuclear programme.
Surge of support
Final results are expected later in the day.
Preliminary figures began to emerge early on Saturday after a delay of several hours.
Hassan Rouhani, a cleric backed by moderate and reformist figures, has taken an outright lead, leaving his main rival, Mr Qalibaf – the current Tehran mayor – a long way behind.
Mr Rouhani has been the supreme leader’s representative in various posts over the last 30 years.
The cleric, a moderate conservative, could line up all moderate and reformist figures plus many moderate conservatives and forge unity among them.
If he becomes Iran’s new president, the country could witness an age of moderation in the next four years.
Officials at the election headquarters said that with more than 16,716,937 votes counted so far – 16,166,392 of them valid:
Hassan Rouhani has 8,439,530 (50.5%)
Mr Qalibaf is in second place with 2,560,383
Mohsen Rezai lies third with 2,101,330, followed by Saeed Jalili on 1,890,462.
State TV channels said the winning candidate needed more than 50% of all ballots cast, including invalid ones, Reuters news agency reported.
Voting was extended by five hours on Friday evening to allow more people to cast their ballots. Iranian Press TV said turnout was 80%.
Although all six candidates are seen as conservatives, analysts say Mr Rouhani – a 64-year-old cleric often described as “moderate” who has held several parliamentary posts and served as chief nuclear negotiator – has been reaching out to reformists in recent days.
The surge of support for him came after Mohammad Reza Aref, the only reformist candidate in the race, announced on Tuesday that he was withdrawing on the advice of pro-reform ex-President Mohammad Khatami.
Mr Rouhani thus went into polling day with the endorsement of two ex-presidents – Mr Khatami and Akbar Hashemi Rafsanjani, who was disqualified from the race by the powerful Guardian Council, a 12-member body of theologians and jurists.
However, Mr Rouhani faced a tough challenge from hardline candidates, including Mr Qalibaf – who is seen as a pragmatic conservative – and nuclear negotiator Saeed Jalili – who is said to be very close to the Supreme Leader, Ayatollah Ali Khamenei.
The other three candidates are Mohsen Rezai, a former head of the powerful Revolutionary Guards, former Foreign Minister Ali Akbar Velayati, and former Telecommunications Minister Mohammad Gharazi.
Earlier, Interior Minister Mostafa Mohammad Najjar told state TV that any presidential candidates unhappy with the results would have three days to lodge complaints to the Guardian Council.
After the last presidential election in June 2009, millions of Iranians took to the streets to demand a re-run, when the supreme leader dismissed claims by the three defeated candidates of widespread fraud.
Two of them, former Prime Minister Mir Hussein Mousavi and senior cleric Mehdi Karroubi, became leaders of a nationwide opposition known as the Green Movement, after its signature colour.
They were placed under house arrested in February 2011 when they applied to stage a protest in support of the anti-government uprisings which were sweeping the Arab world. They are still being detained.
Many reformist newspapers have been shut down, access to the internet and foreign broadcasters has been restricted, and journalists have been detained.
On Thursday, the BBC accused the Iranian authorities of “unprecedented levels of intimidation” of BBC employees’ families.
It said Iran had warned the families of 15 BBC Persian Service staff that they must stop working for the BBC or their lives in London would be endangered.
Tehran has so far made no comment on the allegation.