“We don’t want our country to be trapped in a no-hijab situation, with no discipline,” Moghaddasi said, clutching her black hijab with one hand. “We will only accept Ahmadinejad.”

Moghaddasi was one of tens of millions of Iranians who crowded to the polls to take part in what is widely seen here as a referendum on the hardline policies of President Mahmoud Ahmadinejad. Turnout appeared to be extraordinarily high, with long lines forming outside some polling stations well before they opened at 8 am.

Polls closed at 7 pm, after election officials extended voting for an hour due to the high turnout. Early results are not expected until 12 hours after the polls close. The strong showing appeared to be driven in part by a broad movement against Ahmadinejad that has spurred vast opposition rallies in Iran’s major cities over the past few weeks. Many reform-oriented voters stayed away from the polls in 2005, and now say they are determined not to repeat the mistake. Most say they support Mir Hussein Moussavi, a moderate and former Prime Minister who is the leading opposition candidate.

There are four candidates in the race, and if none wins more than 50 per cent of the vote on Friday, the top two contenders will compete in a runoff a week later. Most analysts have assumed that the election will go to a second round, but in recent days, the extraordinary public support for Moussavi has led to predictions that he could win the presidency in the first round on Friday.

The other contenders are Mehdi Karroubi, a reformist cleric; and Mohsen Rezai, a conservative and the former commander of Iran’s Revolutionary Guards.

Iran’s President is less powerful than the supreme leader, Ayatollah Ali Khamenei, who has final authority over affairs of the state. But the President wields great power over domestic affairs.

As voting began on Friday morning, journalists gathered to watch Ayatollah Khamenei cast his vote at a mosque near his home in southern Tehran.

Just after 8 am, a set of brown curtains opened and the leader emerged, a gaunt 69-year-old with a long white beard, a black turban and a black clerical gown draped around him.

Khamenei presented his identity papers to an official and cast two ballots — one for President, and another for the Assembly of Experts, the 86-member body of senior clerics that appoints and can remove the supreme leader.

“I am hearing about a vast participation of people, and I hear there are even gatherings at night,” Khamenei said. “This shows the people’s awareness.”

The supreme leader also warned about election-day rumours, saying text-messages were being sent around claiming to represent his view on the election, and said they were lies spread by “unhealthy individuals with bad intentions”.

Khamenei has not endorsed any one candidate, but offered a description of the ideal candidate that sounded very much like Ahmadinejad. However, he met for three hours on Thursday with Ali Akbar Hashemi Rafsanjani, a powerful cleric and former President, who heads the Assembly of Experts. Some analysts say Rafsanjani’s lobbying efforts could reduce Ahmadinejad’s freedom to bring out voters.

A number of voters interviewed at the polls on Friday seemed anxious about the possibility of vote-tampering.

“I put one name in, but maybe it will change when it comes out of the box,” said Adel Shoghi, 29, who voted at a mosque in southern Tehran. Shoghi said he favoured Moussavi because Iran needed more civic freedoms, and because Ahmadinejad had worsened Iran’s pariah status internationally, making life hard for Iranians who travel.

Half an hour later, Moussavi arrived at the mosque to cast his vote. He delivered a brief speech.

“This is a golden opportunity for us,” he said. “All this unity and solidarity is the achievement of the revolution and the Islamic republic.” He left soon after, with his admirers in the courtyard still chanting, “Hail to Muhammad, the perfume of honesty and sincerity is coming.”

Ahmadinejad voted at another mosque, in southeast Tehran.

Outside a polling station in an affluent area of north Tehran, women stood in line wearing colourful headscarves, designer jeans, and sunglasses.

Opposition leaders say Moussavi, if elected, would have the advantage of a powerful popular movement behind him. Women have become a potent force in this campaign for the first time in the Islamic republic’s 30-year history.

Moussavi broke with precedent by campaigning alongside his wife, Zahra Rahnavard. Campaign rallies for Moussavi often seem to include more women — who make up half the voters in Iran — than men.

In April, a number of secular and conservative women’s groups joined forces and submitted a list of demands for greater rights from Iran’s next President.

“In the last elections, most people like us didn’t vote,” said Ava Bab, a 24-year-old. “But we saw our situation is getting worse, so we decided to put our hands together.”