The clampdown aims to deny the country's opposition a vital means of communication as it seeks to maintain momentum with periodic demonstrations coinciding with state-sponsored events.
"You [the authorities] do not tolerate the student day rallies. What will you do on the following days?"
Mirhossein Mousavi, opposition leader
Hundreds of thousands of people demonstrated in June in the wake of the re-election of Mahmoud Ahmadinejad as president, claiming that the Iranian authorities had rigged the vote.
Dozens were killed in clashes with security forces and hundreds more were detained by the authorities.
Mirhossein Mousavi, the main rival to Ahmadinejad in the elections, said on his website that the reform movement was still alive despite pressure from the clerical establishment.
"Let's say you suppressed students and silenced them. What will you do with the social realities?" his Kaleme website quoted him as saying, referring to wide arrests of students in Tehran and other cities in the past few days.
"You [the authorities] do not tolerate the student day rallies. What will you do on the following days?" Mousavi said, suggesting that street-protests will continue.
Following the restrictions on the media, Akbar Hashemi Rafsanjani, a former Iranian president and opposition member, criticised the country's rulers of being intolerant of dissent.
"The situation in the country is such that constructive criticism is not accepted," he told students in the northern city of Mashhad on Sunday, the ILNA news agency reported.
"Those who demonstrate or protest must express themselves through legal means. Leaders must also respect the law."
Rafsanjani called on Iran's political groups to work together to "create a climate of freedom which will convince the majority of people and erase ambiguities".