When Abdolkarim Soroush tried to give a lecture three weeks ago, several thousand students turned out to hear him. But the bearded, balding 52-year-old philosphy professor wasn't even allowed to enter the lecture hall. Ansar-eHizbullah - military religious fundamentalists who pay absolute loyalty to the Supreme Leader and Guide of the Revolution, Ayotollah Ali Khamenei - attacked Dr Soroush's car.
"I managed to escape," he said. "In the end, I gave my speech on the telephone, over loud speakers. It was quite an innovation. They might even prevent me doing that next time."
Dr Soroush has encountered similar violence often since he began questioning the God-given right of mullahs to rule the Islamic Republic. The election of a moderate cleric, Hojatolislam Moh amed Khatami, to the presidency last May, has brought the country to "a period of turbulence and tension", he said.
"Khatami's enemies are very determined to topple him before his [four-year] term finishes."
Street demonstrations could lead to violent clashes between moderates and conservatives, Dr Soroush fears. "The defeated wing, the bad guys, are waiting for violence," he said. "In violence they can thrive. The main thing Mr Khatami must avoid by all means is radicalisation."
His criticism of the Velayat-eFaqih, the theocracy established by the late Ayatollah Khomeini, has made Dr Soroush the formost enemy of Iranian religious conservatives.
His appeal for greater intellectual freedom and respect for human rights in Iran have led western diplomats in Tehran to take up the case of dissidents sent to labour camps in the former Soviet Union.
"I have been banned from making public statements, teaching in universities or being quoted by newspapers," he says, adding bitterly: "Sometimes the newspapers have the courage to print my name."
His passport was confiscated last July, and restored to him only in October after an international campaign on his behalf. But Dr Soroush's ideas are spreading. "I receive letters from remote parts of the country," he says "I have recorded 2,000 speeches, and they are widely circulated."
He is often compared to the Protestant reformer, Martin Luther. "Martin Luther is not a hated person in Iran - maybe among Catholics he is hated - but the idea of `protest' is not disliked here. Reformism is a very serious thing for us."
In Tehran, Dr Soroush leads a semi-clandestine existence; his colleagues will not make appointments or say his name on the telephone for fear of sentimentalist wire-tappers, and arranging to see him is a long and haphazard undertaking.
We talked in his book-lined study in Tehran's Academy of Philosophy. "I come here rarely, so that people should not know my movements," he said.
President Khatami has borrowed many of Dr Soroush's ideas, notably the concept that Islam should be about rights rather than duties.
The philospher now criticises the president for failing to act quickly and decisively enough, even if he understands the difficulty of Mr Khatami's position. The two men have had no contact since last May's election.
"His enemies definitely want to associate Khatami with me," Dr Soroush says, "because of my ill reputation, my ideas about Velayat-e-Faqih, about freedom and the clergy. They would like to use the association between us to attack both of us. I avoid him, and he avoids me."
Dr Soroush has been invited to lecture at the British Academy of Arts, but he does not know if the conservatives who still control Iran's security apparatus will allow him to leave the country.
"I have to try," he said. "I have to test it. We are immersed in a sea of uncertainties. When I'm in the air, in the plane, then I will believe it."
President Khatami has appointed a moderate politician as his government spokesman. The Minister of Culture and Islamic Guidance, Mr Ataollah Mohaj erani, has called for dialogue with the US and has eased censorship in the press and cinema.