Deldar's lawyer has said Deldar was conditionally released after serving half of his 15-year sentence.
Radio Farda reports that Deldar had been originally sentenced to death but his sentence was later commuted to 15 years in prison.
Following the 1999 unrest scores of students and activists were arrested. Most of them have been freed, but several, including well-known student activist Ahmad Batebi, remain in custody.
(with material from IRNA)