Ali Khamenei, Iran's supreme leader since 1989, was killed when the United States and Israel launched air strikes on the Islamic republic on February 28.

His son, Mojtaba, was named his successor a week later.

Mojtaba survived the same attack that killed his father, but was wounded, according to various sources who have spoken on the issue.

On Thursday, he issued his first statement since being named, a written declaration read out on state TV.

"The death of Ali Khamenei... marked the beginning of the end of the absolute rule of the clergy," Mohammad Mohaddessin, the chairman of the foreign affairs committee of the National Council of Resistance of Iran (NCRI), told reporters in Paris.

The appointment of Mojtaba Khamenei was a move "effectively transforming the religious dictatorship into a hereditary monarchy", he added.

The NCRI is the political wing of the People's Mujahedin (MEK) organisation, which is banned in Iran and initially supported the 1979 revolution that ended the Iranian monarchy but then fell out with the new rulers.

Mojtaba Khamenei was for years seen as a hugely powerful -- if low profile -- figure working behind the scenes in his father's office.

Mohaddessin said that in that function, Mojtaba Khamenei oversaw not just repressive elements in the security forces but also the "economic system of the regime" via foundations more powerful than those run by the government.

Mojtaba acted as Ali Khamenei's "deputy" although his father, as a "master of deception", never made this into an official position, he added.

"Mojtaba Khamenei's criminal role has long been known," Mohaddessin said.

With his appointment, "the regime's power base has become even narrower", relying even more on the intelligence apparatus and the Revolutionary Guards, the ideological army set up to ensure the survival of the revolution, said Mohaddessin.

"He lacks the authority of his father," a close comrade of revolutionary founder Ruhollah Khomeini, he said.

Mohaddessin said that the Revolutionary Guards are now the "force behind Mojtaba Khamenei" and had ensured his election by the Assembly of Experts clerical body, which was divided over his naming in a "rift at the top of the system".

"The Revolutionary Guards pushed the others to approve the son of Khamenei. They have the upper hand in the regime," he said.

The NCRI is one of several opposition groups calling for an end to the clerical system, notably alongside the monarchists under Reza Pahlavi, the son of the deposed shah.

However both factions fiercely oppose each other.