The election could be the closest presidential race since the 1979 Islamic Revolution. None of the seven candidates is expected to get the 50 percent support needed to win outright, meaning the two top vote-getters will likely meet in a runoff vote.
But Nagi Hassani, a 49-year-old shopkeeper from western Tehran, wasn't taking any chances that he might get lost in the voting crowds and turned up to vote one hour before the polling stations opened.
He waved his birth certificate at an Associated Press reporter outside a polling station in western Tehran saying "I want to be the first one to vote."
Iran's official television showed long lines outside polling stations but there was no independent indication of the early voter turnout.
A smiling outgoing President Mohammed Khatami, who disappointed Iran's young who put him in power in two successive elections by backing down on promised reforms and buckling to pressure from the hard-liners among Iran's powerful clerics, told a press conference that he hoped for a high voter turnout. He said it would strengthen both stability and democracy in Iran.
"I hope the next president will come with development in mind," he said.
The outcome carries added significance since the next president will influence Iran's negotiations with the West over its nuclear program, and its role as a patron of the Shiite Muslim majority in neighboring Iraq.
The front-runner, Ayatollah Hashemi Rafsanjani, who was president from 1989-1997, has portrayed himself as the most experienced figure to handle the sensitive nuclear talks.
Washington claims Iran seeks nuclear arms, but Iran say the program is only for energy production. "I hope we have a fair election, free of manipulation," Rafsanjani said.
Activists opposing Iran's theocracy called for an election boycott. But the nation's ruler, Supreme Leader Ayatollah Ali Khamenei, also urged the 46.7 million eligible voters to flood the 41,000 polling stations to silence critics, led by the United States and pro-reform forces in Iran.
There are another 254 polling stations in other countries where millions of expatriate Iranians can vote.
"They don't want an Islam to have a real democratic system," said Khamenei, who directs the non-elected
Rafsanjani is expected to receive about 45 percent of the vote, short of the mark needed to claim victory, according to campaign manager Mohammed Atrianfar.
Second place would appear to be a contest between a Khatami protege, former Culture Minister Mostafa Moin, 54, and a former police chief, Mohammad Bagher Qalibaf, 44, who appeals to conservatives.
But they share one trait: seeking to become the first non-cleric president since Mohammad Ali Rajai was assassinated in 1981.
Moin, the leading reformist candidate, could be hurt by a boycott called by students who are disillusioned about the prospect of change.
Ray Takeyh, a senior fellow at the Council on Foreign Relations in New York, said the campaign "has brought out some of the fault lines within the conservative camps."
Clerics warned the four conservative candidates that they risked splitting the hard-line vote, but it was only on Wednesday that one of them, Mohsen Rezaei, decided to withdraw.
The conservative vote will still be divided among Qalibaf, former radio and television chief Ali Larijani, and Tehran Mayor Mahmoud Ahmadinejad.
Security forces were on high alert, but there were no immediate reports of unrest or violence. Last Sunday, a bomb killed eight people in the southwestern oil city of Ahvaz. Polls opened at 9 a.m. and were scheduled to close at 7 p.m., but voting hours have been extended in past elections.
Millions of Iranians abroad also will vote at diplomatic sites and other centers, including more than 30 stations around the United States.
"This is more than just who will be president," said Saeed Hajjarian, a top adviser to outgoing President Khatami. "This is how Iran will proceed in a very delicate time for this region."
US President George W. Bush said Thursday in Washington that Iran's election results will mean little because real power rests with the clerics "who suppress liberty at home and spread terror across the world."
More than 20,000 police were deployed around Tehran for the vote and large security contingents were posted in other major cities.
A previously unknown pro-Arab group claimed responsibility for blasts in Ahvaz on the Iraqi border. Two others died in explosions in Tehran the same day, but there's been no claim of responsibility.