Iranians streamed to polling stations on Friday in a hotly contested election and a senior ally of Mirhossein Mousavi said the moderate candidate was on track to defeat hardline President Mahmoud Ahmadinejad.
An Ahmadinejad adviser dismissed the claim as psychological war and said the outcome was impossible to predict.
They were speaking about three hours before polling stations were officially due to close at 6 p.m. (1330 GMT), even though state television said voting was likely to be extended due to an unprecedented turnout.
A victory for Mousavi might help ease tensions with the West, which is concerned about Tehran's nuclear ambitions, and improve chances of engagement with U.S. President Barak Obama who has talked about a new start in ties with Tehran.
Sadegh Kharazi, an ally of the former prime minister, told Reuters that surveys made by reformers showed Mousavi was getting enough votes to win outright in the first round.
I can say that based on our surveys ... Mousavi is getting 58-60 pct of the vote and we are the winner, he said.
Ahmadinejad adviser Ali Akbar Javanfekr said in response: This is a psychological war they launched to influence voting.
Long queues formed at voting centers in the capital Tehran and elsewhere and the Interior Ministry said it expected a turnout of more than 70 percent. Some people said they had waited for more than two hours to cast ballots.
Turnout is unprecedented, said election commission chief Kamran Daneshjou.
A high turnout could indicate the participation of many pro-reformers who stayed away when Ahmadinejad won a surprise victory four years ago on a pledge to revive the values of the 1979 Islamic revolution.
The election has generated interest around the world with policymakers looking for signs of a change of approach by Tehran.
For Iranians the election is a chance to pass judgment on Ahmadinejad's four years in office, particularly his management of the Islamic Republic's oil exporting economy, which is suffering from high inflation and unemployment.
The campaign was marked by mudslinging, with Ahmadinejad accusing his rivals of corruption. The said he was lying about the state of the economy.
Preliminary results are expected early on Saturday. If none of the candidates win 50 percent of the votes, a run-off will be held on June 19 between the two front-runners.
(Additional reporting by Hossein Jaseb and Hashem Kalantari; writing by Fredrik Dahl and Dominic Evans; editing by Richard Balmforth)