Iranians vote on June 12 in an election that pits hardline President Mahmoud Ahmadinejad against two moderate challengers and one fellow-conservative.
Iran, the world's fifth biggest oil exporter, is grappling with the impact of the global economic downturn and weighing its response to U.S. President Barack Obama's diplomatic overtures.
Following are brief portraits of the Islamic Republic's supreme leader and the four contenders in the poll.
AYATOLLAH ALI KHAMENEI
As supreme leader, Khamenei wields ultimate authority under Iran's system of clerical rule known as "velayat-e faqih," or guardianship by an Islamic legal scholar. Not himself elected by popular vote, his wishes can strongly influence elections, which are conducted within rules set by the clerical leadership.
Khamenei, a harsh critic of the United States, is a conservative who plays a balancing role among Iran's competing power centers. He has rarely championed reformist causes.
The 69-year-old cleric took office in 1989 after the death of Ayatollah Ruhollah Khomeini, who led Iran's 1979 Islamic revolution that toppled the U.S.-backed monarchy.
The election is Ahmadinejad's biggest popularity test since he emerged as the surprise winner of the 2005 presidential race.
A former Revolutionary Guard, his anti-Israel rhetoric and defense of Iran's nuclear program cause alarm in the West. U.S.-Iranian tensions have worsened during his term.
Critics at home blame him for disappointing economic growth and rising prices, but his promises to share out Iran's oil wealth more fairly still resonate with the poor. Failure to gain re-election could spell the end of his political career.
Ahmadinejad, 53, Iran's first non-clerical president in more than 25 years, has at times won public support from Khamenei, who urged voters in May to choose an anti-Western leader.
Mousavi, 67, is viewed by many moderates and even some conservatives as Ahmadinejad's strongest challenger for the presidency. He won a name for competent economic management when he was prime minister during Iran's 1980-88 war with Iraq.
That post was abolished in 1989 and Mousavi, an architect and painter, has stayed out of the political frontline since then. He backs a conciliatory foreign policy toward the West.
As war-time prime minister, Mousavi defended state control of the economy, but now advocates more liberalization as the best way to tackle problems such as inflation and unemployment.
A moderate cleric and strong critic of Ahmadinejad, Karoubi also ran in the 2005 presidential poll. He has renewed a pledge to give shares of Iran's oil earnings to every Iranian over 18.
Karoubi, an activist in the Islamic revolution, joined the reformist camp when fellow-cleric Mohammad Khatami was president from 1997 to 2005. However, while he was parliament speaker from 2000 to 2004, he sometimes buckled to pressure from religious and security hardliners who blocked Khatami's reforms.
Karoubi, 72, favors better ties with the United States, often dubbed the "Great Satan" in Iran. But he also follows the official line that calls for U.S. policy concessions upfront.
Rezaie, born in 1954, may get support from conservatives opposed to Ahmadinejad, thus splitting the conservative vote, but few analysts expect him to win the election.
He commanded the Revolutionary Guard during the 1980-88 war with Iraq and now serves as secretary of the Expediency Council, which mediates in legislative disputes.
Rezaie has promised to pursue Ahmadinejad's ideological path, but to take a more moderate foreign policy line. He advocates privatization to invigorate the oil-dependent economy.
An international arrest warrant was issued against Rezaie in 2006 for alleged involvement in the 1994 bombing of a Jewish cultural center in Buenos Aires. He denies the charge.
(Writing by Parisa Hafezi; Editing by Alistair Lyon)