Dozens of Tehran University professors appealed to Iran's supreme leader to halt the ongoing violence against opposition protesters, a pro-reform Web site reported Monday. The letter to Ayatollah Ali Khamenei - signed by 88 professors - was the latest daring challenge to the Iran's clerical leadership. The letter called the attacks a sign of weakness in the ruling system and demanded punishment for those who beat up students. It also urged Khamenei to order arrests over the hard-line crackdown, which intensified after protesters began chanting slogans against the supreme leader. "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. ... Nor is beating up students and their mass imprisonment," the letter read.