Mr. Ruholamini said he had tried for days to find his son, who was arrested in Tehran on July 9. Finally he was directed to the morgue, where he found his son’s body, brutally beaten, his mouth “smashed,” according to an account by a retired senior Revolutionary Guards commander that was posted on various Iranian Web sites and blogs. The report said that Iranian newspapers refused to publish the account.

In another surprising turnabout on Saturday, the head of Parliament’s intelligence committee told the semiofficial Fars news agency that the intelligence minister had changed his mind about broadcasting videotapes of some detainees confessing to plotting a revolution. The opposition and human rights groups say such confessions were wrung from people under duress.

The opposition leaders’ open letter used language that was angry and blunt.

The letter called the government’s crackdown “reminiscent of the oppressive rule of the shah,” who was toppled in Iran’s 1979 revolution. It denied and deplored the accusations of a “velvet revolution,” and criticized the state media and the use of “show confessions.” It even accused the authorities of violating the precepts of the Islamic republic and its revered founder, Ayatollah Ruhollah Khomeini. “The organizations behind this plot have brought us to a point where being a member of an election campaign has become an unforgivable crime,” it said, “even though on repeated occasions the late founder of the Islamic republic and the most distinguished officials of this system have approved and encouraged it.”

The letter represents the latest in a series of stark challenges to Ayatollah Khamenei, who has dismissed claims of election fraud. But it seemed unlikely that the letter would win the support of many of Iran’s senior religious figures, analysts said. And it is not clear what they could do to influence Ayatollah Khamenei.

Some prominent clerics have already made clear that they support the opposition’s claims of fraud and injustice. Only one of the nine men who hold the rank of marja-e-taqlid, or source of emulation, has congratulated Mr. Ahmadinejad on his victory. Three others have condemned the violent crackdown that followed.

But few would dare to openly cross Ayatollah Khamenei. Iran’s clergy is less independent than it was before the revolution, and many analysts say it is growing less powerful.

Mr. Karroubi, the cleric and presidential candidate, also sent a separate letter to Iran’s intelligence chief saying the crackdown on protesters has been worse than Israel’s treatment of the Palestinians, The Associated Press reported.