**Hundreds of Iranian police have surrounded Tehran University to try to stop opposition supporters protesting at an annual rally, reports say.**Authorities had already banned foreign media from the event, which marks the killing of three students in 1953.
Iran has cracked down hard on protests by opposition supporters following a disputed election in June.
On Sunday opposition leader Mir Hossein Mousavi questioned whether the ban would succeed in preventing protesters.
‘Restrictions’
Iranian security forces, including the elite Revolutionary Guards, have warned that they will step in to prevent any attempt to use the event to stage opposition protests.
Opponents of the Iranian government have taken to using officially sanctioned demonstrations to turn out in big numbers and publicise their message.
The annual Students Day event commemorates the death of students during an anti-American protest in 1953.
Before the rally, Tehran residents said that they had been unable to use e-mail and that opposition websites were being more tightly restricted than before.
Thousands have been arrested and dozens killed this year after the disputed election of Mahmoud Ahmadinejad led to the largest street protests since the 1979 Islamic Revolution.