In September 1975, Gaddafi purged the army, arresting around 200 senior officers, and in October he founded the clandestine Office for the Security of the Revolution. In 1976, student demonstrations broke out in Tripoli and Benghazi, and were attacked by police and Gaddafist students. The RCC responded with mass arrests, and introduced compulsory national service for young people. Dissent also arose from conservative clerics and the Muslim Brotherhood, who were persecuted as anti-revolutionary. In January 1977, two dissenting students and a number of army officers were publicly hanged; Amnesty International condemned it as the first time in Gaddafist Libya that dissenters had been executed for purely political crimes.