He may be the bravest student in Iran or an unwitting stooge of the Islamic regime - or both. Either way, Mahmoud Vahidnia has gained instant fame after breaking a taboo by criticizing the supreme leader, Ayatollah Ali Khamenei, to his face.

The 25-year-old mathematics student has been lauded by opposition websites after reportedly telling Khamenei that he had been turned into a "grand idol" who was above criticism. But in a twist demonstrating the inscrutable nature of Iranian politics, the incident has been used by Khamenei's supporters to show how he embraces criticism. Vahidnia has remained unmolested since his 10-minute critique, which condemned the recent brutal post-election crackdown and denounced the state broadcaster, IRIB, for biased coverage. But his most remarkable comments were reserved for Khamenei himself.

"I don't know why in this country it's not allowed to make any kind of criticism of you," he told Iran's most powerful cleric, who has the final say in all state matters. "In the past three to five years that I have been reading newspapers, I have seen no criticism of you, not even by the assembly of experts (a clerical body with the theoretical power to sack the leader). I feel that if this doesn't happen this situation will lead to discord and grudge."

Vahidnia, who achieved nationwide recognition two years ago by winning Iran's annual mathematics Olympiad, made his remarks at a meeting between Khamenei and the country's scientific elite. They came after the supreme leader asked at the end of a question-and-answer session if anyone else wanted to speak. He chose Vahidnia after seeing him being pushed down by officials when he stood to ask a question.

Referring to the post-election crackdown sanctioned by Khamenei, he asked: "Wouldn't our system have a better chance of preserving itself if we were using more satisfactory methods and limited the use of violence only to essential circumstances?"

Although state TV cameras were present, the criticisms only came to light when they were highlighted on Khamenei's own website and by Alef, a fundamentalist site. Both carried accounts showing Khamenei responding calmly.

"Don't think that I'll be unhappy to hear such statements. No, I would be unhappy if such statements are not made," he said. "About lack of criticism of the leader, you go and tell them to criticize. We have not said that no one should criticize us … I welcome criticism. There is criticism and there is a lot of it. And I receive it and I understand the criticism."

The exchange has been seized on by pro-regime media as a demonstration of the leader's tolerance. The hard-line Keyhan newspaper, whose editor-in-chief is appointed by Khamenei, reported it under a headline reading, The Revolutionary Leader's Fatherly Response to Critical Youth.

Some opposition websites suggested that Vahidnia had been arrested by intelligence agents while other reports asked whether he had been a plant set up by regime officials. Vahidnia scotched both suggestions in an interview with Alef, in which he asked "society and elites not to spread rumors".

Under Iranian law comments deemed insulting to the supreme leader carry possible prison sentences, although in practice critics are often not arrested immediately. Ahmad Zeidabadi, the head of Iran's leading student movement, Tahkim-e Vahdat, published an open letter critical of Khamenei in 2007 but was only arrested in the round-ups that followed last June's disputed presidential election.