Risking expulsion and possible arrest, 88 professors at Tehran University signed a letter on Monday calling on Ayatollah Ali Khamenei, the country’s supreme religious leader, to end the use of violence against protesters, saying it was a sign of the government’s weakness.

“Nighttime attacks on defenseless student dormitories and daytime assaults on students at university campuses, venues of education and learning, is not a sign of strength,” the letter read in part. “Nor is beating up students and their mass imprisonment.”

An opposition Web site, Jaras, reported that in another open letter released Monday, five leading opposition figures from outside the country called for the resignation of President Mahmoud Ahmadinejad, free elections, release of political prisoners, greater freedom of speech and an independent judiciary.

The letters did nothing, however, to deter the authorities, who continued a recent crackdown on the opposition with the arrests of dozens of activists and student leaders around the country, Jaras reported. A prominent reporter, Bahman Ahmadi Amoui, who was arrested in June, was sentenced to seven years and four months in prison and 34 lashes, the Web site said.

But the letters did challenge the authority of Ayatollah Khamenei, something that was unthinkable and punishable before the protests that broke out in June after what the opposition calls a fraudulent presidential election.

The five opposition figures abroad singled out Ayatollah Khamenei and warned him that he “would be accountable for the consequences” if the authorities refused to take the steps they recommended to mend the damage done not only over the past seven months, but for many years before.

“The hatred and resentment that has built up against the regime in the past three decades has deep roots,” their letter said. “The discontent has a great destructive power and can unleash a vast wave of violence throughout society.”

The letter was signed by Akbar Ganji, an opposition journalist in New York; Mohsen Kadivar, a senior cleric now at Duke University; Abdolkarim Soroush, a scholar and philosopher at Harvard; Abdolali Bazargan, the son of the first prime minister of the Islamic republic; and Ataollah Mohajerani, the culture minister under President Mohammad Khatami.

The opposition continues to demonstrate the power to mount impressive protests in defiance of official warnings and despite the government’s fiercely repressive tactics. During protests Dec. 27 during the observance of the holy day of Ashura, hundreds of thousands of Iranians protested in the face of a bloody crackdown that killed at least eight people and wounded scores more, according to official figures. The opposition claims the death toll was higher.

The government has blamed Western countries, particularly Britain and the United States, for stirring the unrest. On Monday, the intelligence minister, Heidar Moslehi, told state television that “several foreigners” involved in psychological warfare against the government had been arrested during the Ashura protests. He refused to identify them, but said that they had entered the country on Dec. 25 and that their cases would be handed over to the judiciary by Wednesday.

A visit by members of the European Parliament scheduled to begin Thursday was canceled on Monday, after the Iranian authorities blocked the group’s scheduled meeting with senior members of Parliament and opposition leaders, said Barbara Lochbihler, the German head of the delegation.