Amid the turmoil following Iran's disputed presidential election, world attention is focused on massive demonstrations in Tehran after the disputed re-election of President Mahmoud Ahmadinejad.
The president's leading challenger, Mir Hossein Mousavi, has called for a mass demonstration Thursday to mourn those killed in protests.
But the rallies in the streets of the capital are only the most visible aspect of a power struggle being waged by strong personalities within Iran's complex theocratic government.
In theory, neither the post of president nor the personality of the man who wins it should have much effect on a system designed to guarantee that Iran remains under the control of its Islamic religious hierarchy.
Iran's president is subservient to the powerful Supreme Leader, an ayatollah who has the final say on both domestic and foreign policy. Politicians in Iran cannot run for elected office unless they are considered fundamentally loyal to the regime.
So why is there open dissent about the results of the June 12 presidential election? The answer lies in the interplay between the elected and unelected Iranian government institutions and the influential men who run them.
Here's a look at some key centers of power in Iran and the people behind them:
The Supreme Leader
This post was created for Ayatollah Ruhollah Khomeini, the exiled Iranian cleric who came to power after the 1979 Iranian revolution that ousted Mohammad Reza Shah Pahlavi. After Khomeini's death in 1989, the current Supreme Leader, Ayatollah Ali Khamenei, was appointed by a group of Islamic scholars known as the Assembly of Experts.
The Supreme Leader is the final authority on Iran's policies. Although President Ahmadinejad is most closely associated in the West with policies such as Iran's nuclear program, it is Khamenei who has the final say on such matters.
He is also the commander in chief of Iran's armed forces and has the sole power to declare war or make peace. He controls the country's feared intelligence and security agencies.
Khamenei, who was president himself during the 1980s Iran-Iraq War, is regarded as strongly conservative and suspicious of Western influence.
The Assembly of Experts
Under Iran's constitution, this body selects and supervises the Supreme Leader. In theory, it could fire a leader who was unsatisfactory, but there have been only two Supreme Leaders in the 30-year history of the Islamic Republic, and the assembly has never publicly challenged them.
The assembly is composed of 86 Islamic scholars who are publicly elected from a government-approved list of candidates. They serve eight-year terms.
The current chairman of the Assembly is Ayatollah Ali Akbar Hashemi Rafsanjani. Like Khamenei, Rafsanjani was part of the leadership of the 1979 revolution and has held influential positions ever since, including president. He supported Ahmadinejad's pro-reform opponent, Mousavi.
The Expediency Council
An advisory body appointed by the Supreme Leader, the Expediency Council resolves differences between Iran's elected parliament and its unelected Council of Guardians. It also advises the Supreme Leader on government operations.
Ali Akbar Hashemi Rafsanjani was chairman of the Expediency Council, but there are unconfirmed reports that he may have resigned amid the turmoil over the presidential elections.
Two of Ahmadinejad's opponents in the presidential election, Mousavi and Mohsen Rezai, are also members of the Expediency Council.
The Guardian Council
A powerful group that controls Iran's elections, the council decides who can run for president, parliament and the Assembly of Experts, and it supervises the voting. Six of its 12 members are Islamic clerics appointed directly by the Supreme Leader, and six are jurists elected by parliament.
The Guardian Council bases its approval of political candidates on whether they are deemed to support "Islamic values," and it has been criticized for disqualifying candidates that were considered to be too liberal or reform-minded.
The Guardian Council also has veto power over laws passed by parliament.
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