Iranian moderate Mr Akbar Hashemi Rafsanjani, remains the favourite in the Iranian presidential race but is far from sure of winning the closest election in Iran's history.
Mr Rafsanjani (70), bidding to regain the post he held from 1989 to 1997, wants improved ties with the West.
He told CNN the time was right for a "new chapter" in Iran-US ties but that Washington should gain Iran's trust by unblocking billions of dollar of frozen assets.
But polls in the country of 67 million people show the pragmatist, who masterminded arms-for-hostages swaps with the United States in the 1980s, remains short of the 50 per cent support he needs to avoid a run-off against his closest rival.
His nearest challengers are reformist Mostafa Moin (54), who has pledged to tackle human rights abuses, and conservative Mohammad Baqer Qalibaf (43), an ex-police chief rumoured to enjoy the backing of Supreme Leader Ayatollah Ali Khamenei.
An upset Moin victory could not be ruled out and the outcome of a Rafsanjani run-off against Moin or Qalibaf, possibly on June 24th, would be hard to predict, analysts said.
Tomorrow's vote may determine how Iran handles its atomic standoff with the West and whether it sticks to the path of reform initiated by outgoing cleric Mohammad Khatami.
The campaign has broken taboos in the Islamic state, with many of the candidates pledging to resume talks with Washington and some employing roller-blading girls to hand out campaign leaflets to appeal to young voters.
With half of Iranians aged under 25 and a voting age of 15, all candidates have targeted young voters.