The BBC's Sadeq Saba has visited the Iranian holy city of Qom, the spiritual home of Iran's conservative Islamic establishment and the source of some of the most outspoken dissent against the system.
Qom is one of the main centres of Shia learning in the world and many of the country's senior clerics live in the desert city.
It was from the city that Ayatollah Khomeini started his drive to establish an Islamic system of government in Iran, eventually succeeding in 1979.
THE DISILLUSIONED FORMER SUCCESSOR
"Welcome to your house."
This is how I was greeted in English when I met Grand Ayatollah Hossain-Ali Montazeri at his home in Qom.
He told me he had learnt a little English when he was a political prisoner under the Shah. This was my first meeting with one of Iran's highest ranking clerics, also one of the Islamic Republic's outspoken critics.
He had a pleasant smile and a warm presence. He sat behind his desk and invited me to sit next to him in an armchair.
I immediately felt at home. His son Ahmad, also a cleric, and one of the ayatollah's assistants, was filming the meeting and a friend of mine were also present.
Ayatollah Montazeri, who is in his mid-80s, looked a little frail. He told me the medication for his prostate problem made him feel sleepy all the time.
His house is located in the centre of the city near the shrine of Hazrate Masoumeh, one of the most revered female saints in Shia Islam. It is an old and modest house which serves as both his residence and his office.
While I was waiting to meet him I saw dozens of his followers from all over Iran coming to the office to ask his guidance on religious questions.
Hardline supporters of Iran's supreme leader, Ayatollah Ali Khamenei, attacked this house in 1997 when Ayatollah Montazeri questioned the unaccountable rule exercised by the supreme leader.
The damage to the building was extensive and he was lucky not to be physically harmed. He was immediately placed under house arrest which lasted for more than five years.
During our hour long conversation he repeatedly expressed regret for accepting the role of designated successor to the Islamic Republic's founder, Ayatollah Khomeini.
He was given the position a few years after the Iranian revolution because he was seen as the co-leader of the revolution and as somebody who was instrumental in writing the country's new constitution.
I asked him what it was that finally triggered his split with Ayatollah Khomeini, just a few months before the latter's death in 1988.
He said one day one of his associates, a cleric, came to him in panic saying his 13-year-old daughter was in danger of execution on suspicion of supporting an opposition group.
Ayatollah Montazeri said he did his best to prevent the execution but to no avail.
He said he was gradually becoming disillusioned by the human rights abuses under the new government but he emphasised that the killing of that little girl led him to write his famous scathing letter to Ayatollah Khomeini warning him that the behaviour of his intelligence ministry was worse than that of the Shah's secret police.
Ayatollah Montazeri has a large following because of his status as a senior theologian.
He is known as a key reformist who is trying to adapt Islam to the needs of the modern world by issuing progressive fatwas (religious rulings).
I asked him how the Sharia law that sanctions the killing of Muslims who convert to other religions could be justified.
His answer has far-reaching implications for the Muslims around the world.
He said that if a Muslim converted to another religion on his free will and after a careful study, his choice must be respected and his rights protected.
He said the Prophet Muhammad ordered apostates to be killed because his enemies were deliberately converting to Islam and then converting back to their old religions in order to give Islam a bad name.
He emphasised that such a ruling doesn't apply to the modern world and people should be free to choose their religion.
Ayatollah Montazeri strongly criticised the recent parliamentary elections in Iran. He said the mass disqualification of candidates meant that the new MPs were not actually elected by the people but rather appointed by the authorities.
He thanked the BBC for reporting news of his house arrest.
His son Ahmad said the Iranian government intended to erase his father's name from people's memory by placing him under house arrest. But he said thanks to the BBC Persian service, his father's followers and the public in general were kept informed about what was happening to him.
Ayatollah Montazeri said he always listened to the BBC Persian service even when he was the second most important leader in Iran.
At the end of the meeting he gave me a selection of his new books and a CD containing his celebrated speech in 1997 when he questioned the legitimacy of Ayatollah Khamenei's rule.
As I left his house I was stopped by a man who didn't introduce himself. He asked me a few questions about who I was and what I was doing in Ayatollah Montazeri's house.
Ayatollah Montazeri is not the only senior cleric in Qom who does not approve the regime in Tehran.
I met another prominent theologian who emerged as an opponent of the new Islamic government when Ayatollah Montazeri himself was in power.
Grand Ayatollah Sadeq Rouhani was one of the first senior clerics to be placed under house arrest under a direct order from Ayatollah Khomeini just a few years after the Iranian revolution.
He greeted me warmly at his house. Although in his mid-80s he looked remarkably healthy and energetic. Senior clerics in Iran seem to live long lives.
He sat in his chair as I sat on the floor next to him. He was very proud of his writings on Islamic theology and his library of thousands of books arrayed on the bookshelves behind him.
In contrast to Ayatollah Montazeri, Ayatollah Rouhani is a traditionalist theologian and his opposition to Ayatollah Khomeini was from a more conservative viewpoint.
He told me his differences with the new government sharpened when Ayatollah Montazeri was selected as the future leader of Iran by a clerical assembly in the early 80s.
He said he had nothing against Mr Montazeri personally and no leadership ambitions himself.
In his opinion, Ayatollah Rouhani said, the supreme religious leader of an Islamic state should not be selected by an assembly of other clerics, but rather chosen by divine powers.
Ayatollah Rouhani was so angry at the selection of Ayatollah Montazeri that he publicly declared Ayatollah Khomeini's government un-Islamic.
Armed security forces immediately attacked his house in the middle of the night and he was put under house arrest for 15 years.
Ayatollah Rouhani opposed a range of other government policies and disapproves of such things as the playing of chess and listening to music.
In the middle of our meeting, a cleric from Iran's state television station came to seek the ayatollah's opinion regarding a television drama they were planning to make.
He introduced himself as the religious adviser to the TV station and said the programme was about love affair between young Sunni-and-Shia mixed couple in western Iran.
He wanted to make sure that Ayatollah Rouhani wouldn't oppose the programme when it goes on air.
The ayatollah was reluctant to approve the project and the cleric had to leave after more than half an hour of futile attempts to convince him.
Ayatollah Rouhani told me that over the past years, Iran's supreme leader has repeatedly tried to meet him but he has always refused.
He told me that he was the highest Shia authority in the world and nobody, including Ayatollah Sistani in Iraq, could be compared to him.
Grand Ayatollah Yousef Sanei's house was my next destination. I had no prior appointment but when his assistant told him that I was from the BBC he greeted me warmly.
It was evening by this point and Ayatollah Sanei was sitting on a chair in a large room with about 200 of his followers all sitting on the floor and chatting to each other. He told me that he held such gatherings every night.
Ayatollah Sanei is seen as one of the most progressive theologians in contemporary Iran.
He was the highest judicial authority under Ayatollah Khomeini but later decided to move to Qom to devote his life to theological issues.
He has so far kept himself out of trouble with the government by not publicly challenging the authority of the leader - he has probably learnt from the example of Ayatollahs Montazeri and Rouhani.
But the conservative press regularly attack him for his unconventional religious rulings.
A few years ago he caused uproar in conservative religious circles by declaring that women were completely equal to men in all aspects of political and social life and went as far as saying that a woman could even become the supreme religious leader.
The ayatollah's views are in total contrast to his brother's, Ayatollah Hassan Sanei, who as the chairman of an Islamic charity put a bounty on Salman Rushdie's head after his book The Satanic Verses was deemed blasphemous.
CLERICAL WEDDING PARTY
After leaving Ayatollah Sanei I went to a wedding party with a friend. Both the bride and groom's father were clerics. They had hired a modern public hall in the city for the wedding.
I have never been to such weddings and I was curious to know how clerics, their families and friends celebrate weddings.
When I was introduced to the bride's father as a BBC journalist he treated me as a guest of honour and asked me, jokingly, to broadcast the ceremony live on the radio.
Women and men were separated. The groom was the only man allowed to the women's section where the bride was sitting.
When he came to greet us one by one I noticed that he was wearing a Western-style suit with bow-tie and his hair cut was ultra-modern, a style usually disapproved of by the authorities.
The groom's close friends were all wearing bow ties. In fact most of the guests were wearing ties. Ties are normally frowned upon of as a western or imperialist import in Iran. I was one of the few guests without a tie!
Food, fruits and sweets were plentiful - the only thing missing was alcohol.
There was no music apart from two male singers with beautiful voices who sang unaccompanied for part of the evening.
I asked a cleric who was sitting next to me whether the groom's father approved his son's outfit and hairstyle. He said the poor father had no say in the matter.