Although the government has played down the scale of the prison abuses, some detainees’ relatives have come forward recently to confirm them, mostly to opposition-linked Web sites that have provided credible information in the past, including roozonline.com and gooya.com.

Some deaths have been further documented with photographs or videotapes. Hospital officials have described receiving bodies of those killed in protests, with the total far in excess of 20, the government’s initial figure. It is difficult to confirm such reports independently, given the restrictions on reporting in Iran.

The anger has spread from opposition supporters into Iran’s hard-line camp in part because of the case of Mohsen Ruholamini, the son of an adviser to the conservative presidential candidate Mohsen Rezai, who died in prison after a severe beating. A bitter political dispute among conservatives over Mr. Ahmadinejad’s cabinet decisions may also have helped fuel the issue.

The prison abuses have also galvanized the opposition movement, whose leaders asked for permission to hold a mass mourning ceremony on Thursday in honor of those killed since the election. The Interior Ministry on Tuesday refused permission for the gathering, but the main opposition leaders, Mir Hussein Moussavi and Mehdi Karroubi, said they would hold a public ceremony anyway, several Web sites reported.

Thursday is a day of unusual symbolic importance because it will be 40 days since the shooting of Neda Agha-Soltan, a young woman whose death during a demonstration was captured on video and ignited outrage across the globe. The 40th day marks an important Shiite mourning ritual; similar commemorations for dead protesters fueled the demonstrations that led to the Islamic Revolution in 1979.

Questions about the prison abuse have gained more importance in recent days, not only because of the opposition’s public protests but also because the stories have multiplied. One young man posted an account on Tuesday of his ordeal at the Kahrizak camp, which was ordered closed on Monday by Ayatollah Khamenei.

“We were all standing so close to each other that no one could move,” he wrote in a narrative posted online. “The plainclothes guards came into the room and broke all the light bulbs, and in the pitch dark started beating us, whoever they could.” By morning, at least four detainees were dead, he added.

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In another account posted online, a former detainee describes being made to lie facedown on the floor of a police station bathroom, where an officer would step on his neck and force him to lick the toilet bowl as the officer cursed reformist politicians.

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A woman described having her hair pulled as interrogators demanded that she confess to having sex with political figures. When she was finally released, she was forced — like many others — to sign a paper saying she had never been mistreated.

Mr. Moussavi spoke out Monday in unusually strong and angry terms, accusing the government of brutality and irreligion, and warning that its conduct toward the detainees could set off a much greater reaction.

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“They cannot turn this nation into a prison of 70 million people,” Mr. Moussavi said, adding later that “the more people they arrest, the more widespread the movement will become.”

The prisoner release on Tuesday appeared to be the act of a government desperate to defuse the issue, coming just one day after the head of Iran’s judiciary promised that the detainees’ cases would be expedited. Government officials say that of at least 2,500 people arrested in the postelection crackdown, about 150 remain in prison.

In announcing the release, Saeed Jalili, the secretary of the National Security Council of Iran, sounded a defensive note, saying that those still in jail “are people for whom there are documents stating they were in possession of firebombs and weapons, including firearms, and who had caused serious damage to public property.”

But Mr. Mottahari, the lawmaker, said Tuesday that those responsible for the deaths of detainees must also be identified and punished. Others have gone further, saying the prison abuses suggest a government lurching dangerously out of control.

“Those who have turned this society into a police state and have ordered the use of force have to be held accountable,” said Hamid-Reza Katouzian, a hard-line member of Parliament. “The police and the Ministry of Intelligence have told us that they are on the sidelines, and we do not know who is responsible or accountable.”

Mr. Katouzian is a close friend of Mr. Ruholamini’s family, and his comments appeared to reflect personal outrage over that case. But his remarks also echoed a broader, longstanding concern about the Revolutionary Guards and the Basij militia taking over law enforcement functions and acting beyond the knowledge of legislators.

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Senior clerics have also weighed in, warning that tolerating such injustices could endanger Iran’s theocracy.

“The shameful recent events have distressed everyone and been a source of worry for all those who love their country and the Islamic republic,” said Grand Ayatollah Abdul-Karim Mousavi Ardebili, adding a plea for the government to release detainees.

The number of those killed since the election is impossible to determine, and it includes at least a few members of the Basij militia as well as protesters. One human rights group, International Campaign for Human Rights in Iran, said it spoke to doctors in three Tehran hospitals who registered the bodies of 34 protesters on June 20 alone. Other doctors have provided similar accounts and have estimated a death toll of at least 150 based on corpses they saw.

Earlier this month, family members of missing demonstrators were taken to a morgue in southwest Tehran where they reported seeing “hundreds of corpses” and were not allowed to retrieve bodies unless they certified that the deaths were of natural causes, according to accounts relayed on Web sites and to human rights workers.