3.50pm:
Polling time has been extended until 8pm (4.30pmBST) AP confirmed, in another sign of the huge turnout.
3.35pm:
The Ahmadinejad camp claims their man is winning.
"Based on the evaluation of Ahmadinejad's position he is ahead ... with 60% of the votes and we are certain that the election will end in the first round in his favour," Ali Asghar Zarei told Mehr News Agency, according to Reuters.
3.30pm:
The polls should now have closed, but there are reports that voting has again be extending. An anonymous blogger posting on Bazar Dispatch says it took some friends up to two hours to vote.
The posts says:
Today temperature in Tehran rise to 90 F (32'C), warmer than all days before, and yet people were and are standing long lines in shade and under blazing sun. It is an odd image of Tehran: quiet streets, deserted shopping centers and yet crowded mosques, high schools, schools and colleges. It is election day.
An unprecedented number of voters are casting their vote today. The stations were and are packed with lines coming out of the building and going around the corner.
The individuals waiting in them are from all walks of life, there are Chador wearing women, young ladies wearing the latest fashion and make up, there are old people walking with cane and there are young students with their notes and textbooks in their hands, it is a long wait.
One voted told me he and his sister waited for 2 hours and half to cast their votes. There is an air of a united people present that makes one feel romantic.
I voted in Al-Zahra University station in Vanak Village, it took me 45 minutes waiting time. Most people around me were voting for Mr. Mousavi.
With number present today, one wonders if this is going to be a landslide.
3.15pm:
Voting has been extended by one hour Reuters reports. The polls will close in 15 minutes at 7 pm (3.30 BST) because of heavy queues at polling stations, the Interior Ministry said.
3pm:
Omid, a 28-year old English student, says many of his relatives who had
never voted before have done today - just to get rid of Ahmadinejad, writes Ian Black.
He reports visiting many polling stations in the centre and north of Tehran and insists the turnout is far larger than in 1997, when the reformist Mohammed Khatemi won a landslide victory that ushered in a period of liberalisation at home and detente abroad.
Omid's firm conclusion: a Mousavi win today - not in a second round next week. And some of his conservative friends have switched from Ahmadinejad to Rezaei after the president's embarrassing performance on the economy during their TV debate earlier this week.
Ahmadinejad's charts "explaining" his control of inflation and other indices became an instant laughing stock.
2.15pm:
There are still huge queues to vote and the candidates have had to be reminded about the elections rules on campaigning, writes Ian Black from Tehran.
The central elections committee is warning them to stay away from polling stations because their presence might imply they were still campaigning - in breach of the rules.
IRB TV reports from Shahrikord in southern Iran that lines at polling stations seem to be getting longer as the day goes on. It's the same story in Kerman, home to those wonderful fresh green pistachios.
Back in Tehran, my old friend Koroush, who is proud to say he has never yet voted, insists he will not change his mind even this time, when there is so much excitement about possible change.
It was apathy like that on a massive scale that helped bring Ahmadinejad to power in 2005. That was such a shock that the British Embassy here had to hold an internal enquiry to work out why they had been blind-sided. It concluded they just hadn't been listening to the right people. Let's hope they are this time.
2pm:
The Guardian has a wonderful new gallery of images from today's poll. And there's more from Saeed Kamali Dehghan on the turnout and fears of vote rigging:
More than 4 million votes were cast by midday, according to Iran's interior minister, Sadeq Mahsouli.
He also told Iran's state-run TV that there are not enough polling centres due to the high turnout - and that voting might extend until midnight.
There is a lot of concern among Mousavi supporters about the polling centres in Iran's embassy in the United Arab Emirates (home to half a million Iranians), where there are no reformist representatives to monitor voting.
The interior ministry had also increased the numbers of mobile voting booths - which collect votes from small villages, hospitals and other hard-to-reach places - to 14,000, 10 times more than in the last elections.
Many analysts fear the lack of monitoring of mobile polls might also lead to widespread vote-rigging. Pejman Semnani, an 20-year-old Iranian university student I met in a Tehran polling booth, expressed his fears about mobile polls: "Mobile polls are able to change the results, everybody is concerned about the increase in the number of mobile polls, which reformist newspapers say is neither reasonable nor necessary."
1.30pm:
Saeed Kamali Dehghan writes from Tehran:
There may be four candidates in this election, but it's all about Ahmadinejad and Mousavi, with little obvious support for either Mehdi Karroubi or Mohsen Rezaie. Rezaie's website also appears to have been blocked since the morning.
Mehdi Tavasol, a 27-year-old pro-Karroubi voter says: "I'm pretty sure that Karroubi will not win but the important thing is that any vote for Karroubi and Rezaie will help also Mousavi, because it would drag Ahmadinejad's votes under 50% and would increase the possibility of having a second round of voting."
Progressive Iranians have been worried that Karroubi's presence in the election might affect the reformists' chances of victory but today many analysts are arguing that it might actually help him by attracting Ahmadinejad's votes. Kambiz Moradi, a 47-year-old school teacher believes that Karroubi's presence in the election might also attract voters who didn't participate in the last election.
"Rezaie is also a hardliner and fundamentalist but he is not as popular as Ahmadinejad, so his presence in the election would help Mousavi by attracting Ahmadinejad's votes‚" he explains.
Mousavi's campaign website says he will win in at least 21 out of 30 provinces. By contrast, Rajanews.com, a news website affiliated to Ahmadinejad's supporters, says their opinion polls suggest he will gain 61.5% of the total ballot by the end of the day.
1pm:
Turnout is "unprecedented" says Ian Black in his latest audio dispatch from Tehran. He also talks about reports of vote-rigging and Mousavi's pledge of talks with the US.