Iran chooses a new president on Friday in what is emerging as a two-horse race between moderate ex-premier Mir Hossein Mousavi and incumbent Mahmoud Ahmadinejad, whose turbulent four years in office have been marked by a nuclear standoff with the West and deep economic crisis.
The country is gearing up for a tense battle in Friday's election after a campaign of mudslinging and unusually feisty televised debates between the four candidates.
Running alongside Ahmadinejad and Mousavi are reformist ex-parliament speaker Mehdi Karroubi, the only cleric among the candidates, and the conservative former head of the elite Revolutionary Guards Corps, Mohsen Rezai.
But the race has become a straight fight between Ahmadinejad -- the hardline outspoken son of a blacksmith, and Mousavi -- the last man to hold the post of premier who steered the economy during Iran's war with Iraq in the 1980s.
Kamran Daneshjoo, the head of Iran's election committee, said on Monday he expects "record-breaking turnout" among the 46.2 million eligible voters, half of whom were born after the 1979 Islamic revolution.
He predicted turnout would be high "despite the propaganda of the arrogant nations (Western powers) who are undermining the election."
Analysts are still hesitant to pick a winner, suggesting the vote may be a repeat of 2005 when a relatively unknown Ahmadinejad scored a stunning upset in a second-round runoff against heavyweight cleric Akbar Hashemi Rafsanjani.
A runoff will be held on June 19 if no single candidate emerges with 50 percent plus one vote in the first round.
If Ahmadinejad, 52, is defeated, it will the first time a sitting president is ousted after a single four-year term.
Ahmadinejad's 2005 victory swept the conservatives back into the top echelon of Iranian politics after eight years under reformist cleric Mohammad Khatami who failed nevertheless to implement many of his reforms.
Under Ahmadinejad, however, Iran has become increasingly isolated on the international stage because of its refusal to halt sensitive nuclear work and he has enraged the world over his anti-Israel tirades.
He has also come under fire from his three rivals who accuse him of putting Iran in danger with his policies and of "mismanaging" the energy dependent economy, now battling rampant inflation and shrinking oil revenues.
Another key challenge for the winner will be relations with its archfoe the United States, which under new President Barack Obama is seeking to mend three decades of severed ties while at the same time taking a tough line on the nuclear standoff.
Western powers suspect that Iran's uranium enrichment programme is aimed at making atomic bombs, a charge Tehran vehmently denies.
The programme was frozen under Khatami, but Ahmadinejad resumed it after taking office in 2005, calling the suspension "shameful."
Since then he has doggedly pursued the nuclear agenda, despite three sets of UN sanctions, while Iran has regularly boasted of testing new missiles that could reach its foes in the region.
The four candidates in Friday's race were the only ones to survive a screening by the powerful Guardians Council out of a total of 475 Iranians who registered as prospective candidates, including 42 women.
Khatami and several other reformists have thrown their weight behind Mousavi and anti-Ahmadinejad groups are urging voters to turn out in large numbers to ensure his defeat.
"A 50 percent voter turnout is good for Ahmadinejad, but if around 35 million voters come to the ballot boxes, then it is trouble for him," political analyst Mashaallah Shamsolvaezin told AFP.
He said nearly 50 percent of the eligible voters are "silent voters" who are usually against the hardliners and if they vote in large numbers, it "could turn the game" against Ahmadinejad.
Karim Sadjadpour of US-based Carnegie Endowmnent for International Peace said in a recent report that none of the three challengers had generated the type of excitement that Khatami did in 1997.
"Mousavi, in particular, may receive many votes driven by fear of an Ahmadinejad re-election, rather than a great desire to see a Mousavi presidency," he said.
Mousavi, 67, has however been engaged in stormy debates with the incumbent in the election campaign, lambasting his economic policies and even charging Ahmadinejad's government with "undermining Iran's dignity".
OPEC's second largest oil exporter earns 50 percent of its revenues from oil sales, making its economy heavily dependent on volatile crude prices.
Economists have charged that despite the windfall oil revenues in the first three years of his term, Ahmadinejad's expansionist policies are mainly responsible for Iran's chronic economy.
But in Iran it is the supreme leader who has the final say on strategy.
"When it comes to major issues, such as the nuclear portfolio and relations with the United States, Khamenei will continue to have the last word," said Sadjadpour referring to supreme leader Ayatollah Ali Khamenei.
"But that being said... the president is widely thought of as Iran's second most powerful man. He chooses important cabinet positions and helps set the tone on economic and foreign policy."
"He is also the most visible Iranian official both domestically and internationally."