Mr. Moussavi said little in his statement about the mission and activities of the new movement, perhaps to avoid giving pretexts for a further crackdown and to keep its potential membership as broad as possible.

In recent weeks, outrage about the abuse of jailed protesters  including some who died in custody  has spread from opposition members to many conservatives. The controversy has grown even more volatile in the past week, since the reformist cleric and presidential candidate Mehdi Karroubi first raised accusations that some male and female prisoners had been raped. Hard-line clerics and the speaker of Parliament have vehemently denied the claim, and there have been calls for Mr. Karroubi to be arrested, too.

A third session of the mass political trial was set to begin Sunday morning, with 25 new defendants, Press TV reported. Previous sessions have included confessions by prominent reformists whose friends and relatives said they had been coerced through torture. Last week, a French researcher and an Iranian employee of the British Embassy in Tehran were forced to take the stand and apologize for their efforts to report on Iran’s turmoil, prompting angry protests from Britain and France.

In his announcement, Mr. Moussavi countered efforts to portray him as a tool of secular foreigners, affirming his support for institutions like the Revolutionary Guards and the Basij militia, despite the fact that they are widely believed to be in charge of the current crackdown. But he also lashed out at the recent threats aimed at him and his supporters, saying, “Instead of accusing this millions-strong group, you should look to those who have created a poisonous propaganda war that served the interests of the enemy.”

Also on Saturday, Iran’s supreme leader, Ayatollah Ali Khamenei, appointed Sadeq Larijani as the new chief of the judiciary, replacing Ayatollah Mahmoud Hashemi Shahroudi, a conservative. Mr. Larijani, another conservative and member of Iran’s powerful Guardian Council, is a brother of Ali Larijani, the Parliament speaker. The appointment came as Ayatollah Shahroudi’s term ended and does not appear to be related to the recent controversy over prison abuse and prosecutions of protesters.