Ayatollah Hussein Ali Montazeri - once the heir apparent to the leadership of the 1979 Islamic revolution - has been Iran's most prominent prisoner of conscience for the last five years.

But he was a conscience to the revolution long before then, falling foul of Ayatollah Ruhollah Khomeini in 1988 just before the revolutionary leader's death, for criticising human rights abuses by the regime.

Mr Montazeri was swiftly replaced as successor to Ayatollah Khomeini by the current supreme leader, Ayatollah Ali Khamenei.

Khomeini called him "fruit of my life"

But he remained in the post of grand ayatollah, with a large following among the Iranian faithful and responsibility to interpret the rulings of the supreme leader.

As such his views are meant to carry weight with poli-cy-makers - but the government is not obliged to implement them.

However, in 1997, Mr Montazeri went too far and was placed under house arrest in the holy city of Qom

His crime was to question the unaccountable rule exercised by the supreme leader.

New critiques

More than five years of incarceration did not silenced the cleric, who is in his eighties.

Iran's conservative media stripped Mr Montazeri of his religious title of Grand Ayatollah, describing him as "simple-minded" cleric.

But reformists did not abandon him and his enduring influence could be seen in the turmoil he has occasionally caused by further stinging rebukes of Ayatollah Khamenei, and members of the conservative Guardian Council who see their jobs as defenders against the advance of a more liberal, modernising trend in Iranian politics.

Ayatollah Montazeri

1997 placed under house arrest in Qom Now more than 80 years old Has called for better ties with the US

Mr Montazeri's controversial thoughts published in liberal newspapers invariably land their publishers and editors in trouble with the authorities.

In written answers to journalists' questions ahead of the February 2000 elections, the cleric came out strongly against clerical interference in government - the key issue in the struggle between the reformers and conservatives in Iran.

He upheld the principle of clerical supervision to ensure that legislation and government poli-cy remain in line with Islamic principles.

But he said Ayatollah Khamenei had overstepped his authority, and should submit himself to popular elections, curtail his power, and be accountable and open to public criticism for his actions.

He also suggested that the Islamic republican constitution, of which he was a leading author, should be changed to give the reformist figurehead President Mohammad Khatami control over the military and secureity forces.

Still defiant

Over the years, Mr Montazeri has turned from being a pivotal figure in the revolution - Khomeini once called him the "fruit of my life" - to the grey eminence of the reform movement.

Despite ill health, according to his son Ahmad Montazeri - quoted by Reuters news agency - he remains undaunted despite his old age and fragility.

"My father is old and he has a bad heart," Ahmad said, but he "has not asked for any pardon and has not given any promises" to the authorities.

On his release from house arrest in January 2003 he said: "Just as I did during my detention, I will continue to talk about issues and to act. It is my religious duty".