Prisoners, including women and teenagers, were loaded onto forklift trucks and hanged from cranes and beams in groups of five or six at half-hourly intervals all day long. Others were killed by firing squad. Those not executed were subjected to torture. The victims were intellectuals, students, left-wingers, members of the People's Mojahedin Organisation of Iran (MEK), other opposition parties..... Many had originally been sentenced for non-violent offences.....

Link to the original piece:

http://www.economist.com/blogs/newsbook/2012/06/iran-1988