|Karroubi, left, has been one of the most outspoken critics of Ahmadinejad's policies [AFP]
On Friday, June 12, Iran holds its tenth presidential election since the revolution that created the Islamic Republic.
Following a vetting process by the Guardian Council - a powerful legislative oversight body consisting of six theologians appointed by the supreme leader and six jurists nominated by the judiciary and approved by parliament - four candidates have been deemed eligible to stand.
To win the presidency, a candidate must receive more than 50 per cent of the total vote. Otherwise, the contest will go to a run-off vote usually held the following week.
Mahmoud Ahmadinejad, the current president, is running against two reformist candidates and one fellow conservative.
For the first time since the creation of the Islamic Republic in 1981, the race for the presidency is expected to be closely run, with Mir Hossein Mousavi, the frontrunner reformist candidate, gathering a groundswell of support.
The incumbent 52-year-old is seeking a second term as Iran's president.
Prior to his victory four years ago, Ahmadinejad was mayor of Tehran as well as a member of Iran's influential Revolutionary Guards.
He is generally viewed as religiously and politically conservative.
His modest background is one factor behind his appeal with the masses, whom he has pledged to reward with fairer distribution of the country's vast oil wealth.
Ahmadinejad has said he favours dialogue with the US, as long as it is based on mutual respect.
The leading candidate running against President Ahmadinjead is 67-year-old Mir Hossein Mousavi.
He is best known as the country's last serving prime minister during the Iran-Iraq war in the 1980s. During the war he pioneered the country's food rationing and distribution system.
Since then, he has largely been out of politics, heading Iran's Academy of Art which is responsible for safeguarding the country's national heritage.
Mousavi played a key role in the Islamic Revolution and had close ties with the late Ayatollah Khomeini.
As part of his return to politics in this election, he has pledged to change Iran's image abroad.
However, he believes Iran should refuse to back down on its nuclear programme and maintains the country's right to nuclear technology.
In another of his campaign pledges, he has committed to reviewing discriminatory laws against women and stopping the operation of so-called morality policies.
The 72-year-old is a mid-ranking cleric.
He is considered a reformist and heads Iran's National Trust Party, as well as having previously been parliamentary speaker.
Karroubi has been one of the most outspoken critics of President Ahmadinejad's policies, and is a well-known figure on Iran's political scene. He has already been endorsed by several high ranking figures.
His election campaign is based on the slogan "change", leading some to call Karroubi the "reform sheikh".
He has pledged to nationalise the country's vast oil profits, and to redistribute the money to every Iranian over the age of 18 through a system of stocks.
He has also pledged to protect civil rights and improve the status of women in Iran.
The cleric also says he favours dialogue with the international community.
The 55-year-old is the current secretary of the Expediency Council, a major body that arbitrates on disputed legislation, and the only candidate running against Ahmadinejad who is also considered to be a conservative.
Rezai is the former head of the Revolutionary Guards, one of the most powerful paramilitary organisations in Iran.
He is currently on the Interpol wanted list, for suspected links to the 1994 bombing of a Jewish cultural centre in Buenos Aires. He is banned from entering the US and the EU.
Rezai holds a doctorate in economics and says, if elected, his priority will be to "revolutionise" the economy.
One of his main pledges is to greatly increase the role of the private sector and to guarantee an increase in foreign investment.
Rezai has also been critical of Ahmadinejad's comments about the holocaust.
He says he has formulated a step-by-step plan to improve relations with the US, and that he also believes Iran should improve its strategic role in the Middle East by closing ranks with its regional neighbours.