A spokesman for the Guardian Council supervisory body said on Saturday there was no clear winner in Friday's presidential poll.
"There will be a second round on Friday," Gholamhossein Elham told reporters.
He said 20 million out of an estimated 32 million votes cast had been counted, excluding ballots from big cities.
Elham said former president and cleric Akbar Hashemi Rafsanjani was ahead with 20.84%, Tehran mayor Mahmood Ahmadinejad second with 19.71% and reformist cleric Mehdi Karoubi third with 17.57%.
An interior ministry official involved in the counting
told The Associated Press that the vote count he had seen
made it impossible for any one candidate to collect the
required 50% plus one to win.
The election had initially been painted as a one-horse race for veteran Rafsanjani, who is hoping his image as a business-savvy pragmatist with clout has wooed a nation tired of political deadlock, economic stagnation, a nuclear crisis and international isolation.
But he faced a tough challenge from Moin, a leftist who has promised to fill his cabinet with political dissidents and challenge unelected hardliners like supreme leader Ayatollah Ali Khamenei.
Reformist Moin was seen as
Rafsanjani's main challenger
The main hardline contender was ex-police chief Mohammad Baqer Qalibaf, but a late charge by Tehran's ultra-conservative mayor Ahmadinejad and the presence of former state television boss Ali Larijani may have split the conservative vote.
"I envisage a second round run-off between Moin and Hashemi (Rafsanjani). I think the electorate came out in the afternoon and it was mainly for Moin," said Mohammad Ali Abtahi, a reformist and former vice-president.
Mohammad Atrianfar, a close aide to former president Rafsanjani, echoed the prediction of a run-off on either 24 June or 1 July.
But their comments are subject to caution, and the uncertainty is such that even seasoned observers could only make do by describing the election as the tightest in Iran's history.
"The Iranian nation has usually defied predictions," said outgoing reformist President Mohammad Khatami as he voted with an ear-to-ear grin - indicative of his relief that his difficult eight years in office were coming to an end.
Voter turnout is estimated to
have been over 55%
More than 46 million people, men and women above the age of 15, were eligible to vote. Crucially, 70% of the population is under 30 and too young to remember the 1979 revolution.
Voting had been extended by four hours on Friday, with the authorities giving voters plenty of time to carry out what Iran's supreme leader described as a "pious act" of support for his 26-year-old oil-rich Islamic republic.
The government has been hoping to counter widespread apathy and boycott calls from students and prominent liberals such as Nobel Peace Prize winner Shirin Ebadi - who argue that unelected clerics like the deeply conservative Khamenei hold the real power.
"I envisage a second round run-off between Moin and Hashemi (Rafsanjani)"
Mohammad Ali Abtahi,
Former Iranian vice-president
Even though the seven candidates were pre-screened, the contenders have very different outlooks.
Rafsanjani says he is opposed to "extremists", in favour of economic liberalisation and open to talking with US President George Bush - who he once branded a "bird-brained dinosaur".
But the silver-haired cleric, known as Iran's point man in the "Irangate" weapons-for-hostages deal in the 1980s, has been dogged by corruption allegations and his record on human rights.
Former higher education minister Moin was initially disqualified from standing and is clearly a man that entrenched hardliners do not want to see in Iran's number-two job.
He is hoping for an upset similar to Khatami's spectacular 1997 landslide win, even though the wave of popular euphoria that brought Khatami to power has been dampened by his failure to grab the strings of power - let alone pull them.
Qalibaf, a Revolutionary Guards veteran who has reinvented himself as a trendy technocrat, promises a bread-and-butter focus in a country dogged by inflation and joblessness.