Iranians packed polling stations from boutique-lined streets in north Tehran to rural conservative bastions today for a choice that has divided the nation.
At stake is re-election for hard-line President Mahmoud Ahmadinejad or his replacement by reformer Mir Hossein Mousavi who favours greater freedoms and improved ties with the West.
Crowds formed quickly at many voting sites in areas considered strongholds for both candidates.
Mr Mousavi, who served as prime minister in the 1980s has become the surprise hero of a powerful youth-driven movement.
The turnout was massive - with some officials predicting a record-breaking surge. Voting was extended initially by one hour - and then by a total of three hours. Some polling stations that had closed were then re-opened.
There were no reports of unrest or serious problems at the polls. But a top Mousavi aide said some polling stations in north-western and southern provinces ran out of ballots, claiming it was a "deliberate attempt by the government to keep people from voting."
The fiery, month-long campaign unleashed passions that could bring a record turnout. The mass rallies, polished campaign slogans, clever internet use and televised debates more closely resembled Western elections than the scripted campaigns in most other Middle Eastern countries.
In a sign of the bitterness of the campaign, the Interior Ministry - which oversees voting - said all rallies or political gatherings would be banned until after all results are announced, which is likely to be tomorrow.
The outcome will not sharply alter Iran's main policies or sway high-level decisions, such as possible talks with Washington. Those crucial policies are all directly controlled by the ruling clerics headed by the unelected Supreme Leader Ayatollah Ali Khamenei.
But Mr Mousavi has offered hopes of more freedoms at home. If elected, he could try to end crackdowns on liberal media and bloggers and push for Iran to embrace President Barack Obama's offer of dialogue after a nearly 30-year diplomatic freeze.
He favours talks with world powers over Iran's nuclear programme, which critics fear is aimed at making weapons. Iran says it only seeks reactors for electricity.
With the race considered too close to call, a top election official predicted turnout could surpass the nearly 80% in the election 12 years ago that brought President Mohammad Khatami to power and began the pro-reform movement.
A strong turnout could boost Mr Mousavi. He is counting on under-30s, who account for about a third of Iran's 46.2 million eligible voters.
Mr Mousavi's rallies in Tehran drew tens of thousands of cheering supporters, who later spent their nights shouting anti-Ahmadinejad slogans and dancing to Persian pop songs on the streets.
For the first time in Iran, the forces of the web were fully harnessed in an election.
Today dozens of Iranians using Twitter posted messages including one that proclaimed: "For Iran, this is the Day of Decision."
Another said: "Keep my fingers crossed for green wave to win."
Mr Mousavi's stunning rise also has been helped by his popular and charismatic wife, former university dean Zahra Rahnavard, and their joint calls for more rights and political clout for women. Iranian women work in nearly all levels of society - including as parliament members.
But they face legal restrictions on issues such as inheritance and court testimony, where their say is considered only half as credible as a man's.
Iran's elections are considered generally fair, but the country does not allow international monitors. The ruling clerics, however, put their stamp on the elections from the very beginning by deciding who can run. More than 470 people sought to join the presidential race, but only Mr Ahmadinejad and three rivals were cleared.
If no candidate gets a simple majority of more than 50% the election will go to a runoff on June 19 .
Much depends on how many votes are siphoned off by the two other candidates: conservative former Revolutionary Guard commander Mohsen Rezaei and moderate former parliament speaker Mahdi Karroubi.
