Officials have also ordered journalists working for foreign media organisations not to leave their offices to cover the demonstrations.

Iran's beleaguered opposition has sought to maintain momentum with periodic demonstrations coinciding with state-sanctioned events.

The rallies will take place on a day that normally marks a 1953 killing of three students at an anti-US protest. Since the 1990s, the day has served as an occasion for pro-reform protests.

Students are at the centre of the opposition to Iran's clerical regime and its brutal crackdown on demonstrators protesting against what they believed was a fraudulent presidential election in June.

The opposition, which relies on the web and mobile phone service to organise rallies and get its message out, has vowed to hold rallies, the first anti-government show of force in a month. It is not clear if the demonstrations will take place on university campuses or in the streets.

The call went out on dozens of websites run by supporters of opposition leaders Mir Hossein Mousavi and Mahdi Karroubi, both of whom ran in the June 12 election. Most of those sites have been repeatedly blocked by the government, forcing activists to set up new ones.

Internet connections in the capital, Tehran, have been slow or completely down since Saturday. Blocking internet access and mobile phone service has been one of the routine methods employed by the authorities to undermine the opposition in recent months.

The government has not publicly acknowledged it is behind the outages, but Iran's internet service providers say the problem is not on their end and is not a technical glitch.