The Basij militia is an Iranian volunteer force of Islamic government loyalists which is often called out onto the streets at times of crisis to dispel dissent.
The force was originally set up by Ayatollah Khomeini in 1979 as a resistance force during the Iran-Iraq war.
They received limited training and were used for "human wave" attacks, for example being asked to clear Iraqi minefields by walking across them.
The size of the militia is an open question.
Many Iranian officials cite 20m - the number that Ayatollah Khomeini once suggested would be an invincible force - but independent estimates put the force at as little as 400,000.
A 2005 study by the Centre for Strategic and International Studies in the United States, estimated 90,000 full-time, uniformed, active-duty Basij members and 300,000 reservists. There are also believed to about a million affiliates who can could be mobilized if need be.
The Basij-e Mostaz'afin, (literally Mobilization of the Oppressed in Farsi), officially known as the Basij Resistance Force (Nirouye Moqavemate Basij), has branches in every town.
It is commanded by a senior cleric and is an auxiliary arm of the powerful Iranian Revolutionary Guard.
Historically Basijis (militia members) have been pro-regime and the force has seen a revival under the presidency of Mahmoud Ahmadinejad.
In past elections, large numbers of the rank and file in the Revolutionary Guards and the Basij militia were reported to have voted not just for hardliners like Mr Ahmadinejad, but also for reformist candidates.
But there is no doubt that Mr Ahmadinejad still has large support from within the Basij militia, which is made up, in large part, of boys from poor, religious families, often from rural areas, who have benefited from government policies in the last four years.
Aside from being used to quell civil unrest, Basijis are employed as overseers of civilian behaviour, enforcing dress codes, emergency management and the suppression of dissident gatherings.
In the days following the 2009 presidential elections, members of the militia were accused of being responsible for the deaths of seven anti-Ahmadinejad protesters after they fired at a crowd that had attacked a Basij compound.
They were also accused of attacking students at Tehran University and other academic institutions.
Despite usually being seen as beyond scrutiny, the interior ministry agreed to an investigation following a call from the parliament speaker, Ali Larijani.