By Jon Leyne
BBC News, Tehran
The number of young women voting has been striking
It has become an extraordinary day, at the end of what has been an extraordinary election campaign.
As soon as polls opened in Iran, it became clear that the enthusiasm of the last few days has been translated into what is likely to be a huge turnout.
There were queues snaking round the block from many polling stations.
The crowds gathered outside, in segregated lines of men and women. Even as they waited to vote, they continued the spontaneous debate that has been sweeping Iran in the last week.
At one polling station I visited, some voters came up to me, nervous that the government might be trying to rig the election.
They were worried that a bus being used as a mobile polling station was not as well monitored as the main polling centre.
Other voters say the system under which a reference number has to be written by the candidates' name on the ballot paper is confusing.
Much of the mobile phone text message system seems not to be working, a system the opposition had been hoping to use to send back reports from their monitors at polling stations and election counts.
The opposition has complained to the government.
Rumours are sweeping Tehran that some satellite TV stations may have been blocked.
But for the most part election day has continued the good humour of recent days.
One supporter of President Mahmoud Ahmadinejad, a woman in the long black religious chador, made a point of shaking hands with another woman wearing the green colours of the opposition contender, Mir-Hossein Mousavi.
Many of the polling stations are in mosques or other religious buildings.
At the Hosseiniyat Ershad in north Tehran, the number of women, particularly young women, queuing to vote is most striking.
The young voters who have been turning out in force for Mr Mousavi say they want more personal freedom, more opportunities and better relations with the West.
Supporters of Mr Ahmadinejad have praised him for pushing forward the nuclear programme, and say he has earned more respect for Iran internationally.
By mid-morning, the interior ministry announced that already five million people had voted. Voting was extended by at least four hours.
Such a high turnout will make Iranians more confident of the outcome.
They will remember the election in 1997, in which President Khatami defeated a candidate heavily favoured by the establishment.
His victory was so overwhelming it soon became clear that it could not be overturned, even if there had been those trying to do so.
Results are expected to begin coming in during the night. Almost every Iranian you meet is eager for any idea about what is going to happen.