As the situation in Iran becomes increasingly volatile, we take a look at the players in Iranian society.
The Supreme Leader, Ayatollah Ali Khamenei
Iran's Supreme Leader, Ayatollah Khamenei, is the country's most powerful figure.
He appoints the head of the judiciary, six of the 12 members of the powerful Guardian Council, the commanders of all the armed forces, Friday prayer leaders and the head of radio and TV. He also confirms the president's election.
Khamenei was a key figure in the Islamic revolution in Iran and a close confidant of Ayatollah Khomeini, the founder of the Islamic republic. He was later president of Iran from 1981 to 1989 before becoming Supreme Leader for life.
President Mahmoud Ahmadinejad
Mahmoud Ahmadinejad, who has been Iran's president since 2005, was actively involved in the Islamic revolution and was a founding member of the student union that took over the US embassy in Tehran in 1979. But he denies being one of the hostage-takers.
He became the first non-cleric to be elected president since 1981 when he won a run-off vote against former president Ali Akbar Hashemi Rafsanjani in elections in June 2005.
He is a hard-liner both at home - where he does not favour the development or reform of political institutions - and abroad, where he has maintained an anti-Western attitude and combative stance on Tehran's nuclear programme.
Much of his support comes from poorer and more religious sections of Iran's rapidly growing population, particularly outside Tehran.
Mir Hossein Mousavi
The 68-year-old former prime minister stayed out of politics for some years but returned to stand as a moderate in the 2009 presidential poll which returned Mahmoud Ahmadinejad to power amid widespread allegations of vote rigging.
Mir Hossein Mousavi was born in East Azerbaijan Province and moved to Tehran to study architecture at university.
He is married to Zahra Rahnavard, a former chancellor of Alzahra University and political advisor to Iran's former President Mohammad Khatami.
One of his closest associates and backers in this election was Ali Akbar Hashemi Rafsanjani, the former President of Iran who now heads two of the regime's most powerful bodies: The Expediency Council (which adjudicates disputes over legislation) and the Assembly of Experts (which appoints, and can theoretically replace, the Supreme Leader).
The 73-year-old cleric is a veteran politician who served as a member of Iran's parliament for 16 years, and speaker for two terms.
The son of a Shia clerical family from Aligoudarz, south-west of Tehran, he gained his religious credentials in Qom before working as a commercial lawyer. A staunch supporter of Ayatollah Ruhollah Khomeini, he helped to organise protests against the Shah and was arrested and jailed several times in the run up to the revolution.
He came third in the 2005 presidential elections and fourth in 2009, alleging widespread fraud and publicly accusing the regime of rights abuses against protesters in the crackdown that followed.
He currently leads the National Trust Party, which champions greater personal rights and freedoms, the empowerment of women, and tolerance of religious minorities.
He is married to Fatemeh Karroubi, a social activist who led his campaign in Tehran. They have four sons.
Ali Akbar Hashemi Rafsanjani
Ali Akbar Hashemi Rafsanjani, has been a dominant figure in Iranian politics since the 1980s.
Described as a "pragmatic conservative", he is part of the religious establishment, but he is open to a broader range of views and has been more reflective on relations with the West.
Mr Rafsanjani was president for eight years from 1987 and ran again in 2005. He lost to Mahmoud Ahmadinejad in the second round. He has been openly critical of the president since then.
He is still a powerful figure in Iranian politics as he heads two of the regime's most powerful bodies: the Expediency Council (which adjudicates disputes over legislation) and the Assembly of Experts (which appoints, and can theoretically replace, the Supreme Leader). He is also a wealthy businessman.
The Iranian reform movement is a political movement led by a group of political parties and organizations in Iran who supported Mohammad Khatami's plans to introduce more freedom and democracy.
In 1997, Khatami was elected president on a platform of greater freedom of expression, as well as measures to tackle unemployment and boost privatisation. However, much of his initial liberalisations were stymied by resistance from the country's conservative institutions.
He initially stood for election in 2009 but later stood aside and lent his support to Mir Hossein Mousavi.
Other key reformist figures include Mir Hossein Mousavi, Mehdi Karroubi, Mohsen Mirdamadi, Hadi Khamenei, Mohsen Aminzadeh, and Mostafa Tajzadeh.
One of their spiritual mentors, and a fierce critic of the regime, was Grand Ayatollah Hoseyn Ali Montazeri , who died in December 2009.
The Revolutionary Guard and the Army
The armed forces comprise the Revolutionary Guard and the regular forces. The two bodies are under a joint general command.
Iran's Islamic Revolution Guards Corps (IRGC) was set up shortly after the revolution to defend the country's Islamic system, and to provide a counterweight to the regular armed forces.
It has since become a major military, political and economic force in Iran, with close ties to the Supreme Leader, Ayatollah Ali Khamenei, and President Mahmoud Ahmadinejad, a former member.
The force is estimated to have 125,000 active troops. It boasts its own ground forces, navy and air force, and oversees Iran's strategic weapons.
The Guards also have a powerful presence in civilian institutions and are thought to control around a third of Iran's economy through a series of subsidiaries and trusts.
The Revolutionary Guard also controls the Basij Resistance Force, an Islamic volunteer militia of about 90,000 men and woman with an additional capacity to mobilise nearly 1m.
The Basij, or Mobilisation of the Oppressed, are often called out onto the streets at times of crisis to use force to dispel dissent. There are branches in every town.
Clerics dominate Iranian society.
Only clerics can be elected to the Assembly of Experts, which appoints the Supreme Leader, monitors his performance and can in theory remove him if he is deemed incapable of fulfilling his duties. The Assembly is currently headed by Iran's former President Ali-Akbar Hashemi Rafsanjani, who is described as pragmatic and conservative.
Former President Mohammad Khatami accused the clerics of obstructing his reforms and warned against the dangers of religious "despotism".
Clerics also dominate the judiciary, which is based on Sharia (Islamic) law.
In recent years, conservative hardliners have used the judicial system to undermine reforms by imprisoning reformist personalities and journalists and closing down reformist papers.