Formed shortly after the 1979 Islamic Revolution, the Guards answer to supreme leader Ayatollah Ali Khamenei and boast their own ground, naval and air forces.

Working in parallel to the regular army, the corps' mission is to "guard the revolution and its achievements" as well as "to realise holy ideals and to propagate the rule of God's law".

"Their objective is not the promotion of largely secular traditional Iranian nationalism but to promote the religious ideology incarnated in the Islamic republic," said Clement Therme, research fellow at the International Institute for Strategic Studies (IISS).

- Overseas activities -

In the 1980-1988 war with Saddam Hussein's Iraq, many tens of thousands of its members died in battle defending their country.

In recent years the Guards have paid with their blood as they have been involved in "fighting terrorism" in Iraq and Syria, officially acting as "military advisors".

The activities abroad of their elite Quds force -- headed by Major General Qasem Soleimani -- have drawn accusations of dangerous meddling from Washington and its regional allies.

But Iran insists it is an example of regional cooperation aimed at shoring up stability and blocking Western interference.

Estimated to be around 125,000 strong by the IISS, the Guards have faced censure from the US over their role in Iran's nuclear and ballistic missile programmes.

A branch of the corps is in charge of the country's ballistic missiles, and the force also controls the Basij paramilitary volunteer force.

Its naval arm, estimated to be 18,000 strong, is charged with the defence and security of the Gulf, including the strategic Straits of Hormuz.

The current overall commander of the Guards is Major General Mohammad Ali Jafari.

- Business and politics -

Over the decades the Guards' influence has expanded well beyond military affairs and into politics and the economy as they became a powerhouse within Iran's complex ruling structure.

Former cadres entered most branches of power and its construction arm grew into one of the country's top contractors with projects ranging from petroleum to mining.

Many ministers and members of parliament, including current speaker Ali Larijani, served in the corps before entering politics.

The Revolutionary Guards have at times defied sitting presidents.

Reformist Mohammad Khatami was confronted by its tanks when he attended the inauguration of Tehran's international airport in 2004 following a disagreement over control of the facility.

In another incident Khatami received a letter from prominent Guards generals warning "patience is ending" over his policy of social freedoms and detente with the West.

Ultra-conservative Mahmoud Ahmadinejad, his successor and greatly admired by the Guards to begin with, increasingly distanced himself from them as he claimed they were encroaching on his authority.

The trend has continued with the moderate incumbent Hassan Rouhani, who has publicly both criticised and in turn been criticised by the Guards.

Rouhani has tried unsuccessfully to curb the Guards' economic activities, sparking public rows.

But the president was still quick to leap to their defence against Monday's terrorism designation by the US, hailing them for fighting "terrorism".