US President George W Bush has criticised the presidential election taking place in Iran on Friday as ignoring the demands of democracy.
Akbar Hashemi Rafsanjani was president from 1989-97
"Iran is ruled by men who suppress liberty at home and spread terror across the world," he said in a statement released by the White House.
The front-runner is former President Akbar Hashemi Rafsanjani.
However, he faces a tough challenge from two rivals, meaning a run-off vote may be needed for the first time.
Mr Bush criticised Iran for blocking hundreds of reformist candidates from running.
"Power is in the hands of an unelected few who have retained power through an electoral process that ignores the basic requirements of democracy," he said.
Mr Bush said the US was united with the people of Iran when they stood up for freedom.
The voters will be choosing a successor to reformist President Mohammad Khatami, who is barred by law from standing again.
Campaigning ended early on Thursday morning after carrying on through the night. Many young men and women took the opportunity to stay out late and mingle in a relaxed atmosphere not normally permitted in Iran.
With half of the 67 million population under 25 - and 15-year-olds allowed to vote - all the candidates have been appealing to the youth vote.
The seven candidates are a mix of conservatives and reformists. Mr Rafsanjani - who was president from 1989-97 - is seen as one of the most likely to appeal to voters on both sides of the spectrum.
Mr Khatami swept to victory in 1997 on a reformist ticket.
But apathy has taken hold after his programme failed to deliver change in the face of the conservative religious establishment, which is able to veto legislation and political candidates.
Expert projections suggest most votes will be shared between Mr Rafsanjani, former hardline police chief Mohammad Baqer Qalibaf, and reformist Mostafa Moin.
If no-one polls more than 50%, a second round will be held on 1 July.
Mr Moin's supporters say a high turnout would benefit him, but correspondents says many young people and reformists are too disillusioned with the political and economic situation to bother voting.
A day before the election, Mr Khatami warned of an organised dirty tricks campaign.
He did not identify who was behind the interference, which he said included "disruption of gatherings, beatings, illegal pamphlets and spreading lies to ruin candidates' reputations regardless of political inclination".
Mr Rafsanjani has also denounced an alleged dirty tricks campaign against him.
Iran has been rocked by a series of bombings in recent days that left up to 10 people dead.