Police and elite Revolutionary Guards have warned that any "illegal" rally will be fiercely confronted on Monday when the country marks Student Day, commemorating the killing of three students in 1953 under the former Shah.
"All permits issued for foreign media to cover news in Tehran have been revoked from December 7 to December 9," the Culture Ministry's foreign press department said on Saturday in an SMS text message sent to journalists, photographers and cameramen working for foreign media in Iran.
In the past few days, Internet connections in Tehran have been either very slow or completely down. An official at Iran's telecommunications ministry told Reuters that Internet access and cellphone lines would be disabled on Monday.
When the June 12 presidential election returned hardline President Mahmoud Ahmadinejad to power by a wide margin, his reformist opponents cried foul and thousands of Iranians took to the streets in the biggest anti-government demonstrations in the 30-year history of the Islamic Republic.
Authorities deny vote-rigging and portrayed the unrest as a foreign-backed bid to undermine the state.
"Any illegal gathering outside universities will be strongly confronted," said police chief Esmail Ahmadi-Moqaddam, quoted in the Etemad newspaper.
The Paris-based press freedom watchdog Reporters Without Borders said in a statement on Saturday that journalists were facing increasing difficulties in Iran.
"The press freedom situation is getting worse by the day in Iran," it said.
"Journalists who have chosen not to leave the country are being constantly threatened or summoned by the intelligence services, including the intelligence service of the Revolutionary Guards. Some have been given long prison sentences at the end of completely illegal judicial proceedings."
The watchdog said 28 journalists and bloggers were currently detained and urged the authorities to release them.
Several moderate websites have called on people to gather on Student Day near Tehran University campus, where the main state-rally will be held.
So far, defeated candidates Mirhossein Mousavi and Mehdi Karoubi have not announced whether they will join the anti- government protests as they have done in the past.
Reformist website Mowjcamp has warned of the possibility of clashes between security forces and demonstrators.
"Street protests on Monday lack the safety demonstrators enjoy in a state rally … a violent crackdown is expected on that day," said the website.
In September, opposition demonstrators clashed with government backers and police at annual pro-Palestinian rallies.
Security forces also clashed with supporters of Mousavi in Tehran on November 4 during an anti-U.S. rally, used by the opposition to protest against the clerical establishment.
Thousands of reformists, including former senior officials, students, lawyers and activists were arrested after the June vote. Most of have since been freed, but more than 80 people have so far been sentenced to jail terms of up to 15 years and five people have been sentenced to death.
The reformist opposition says more than 70 people were killed in post-election violence. Officials say the death toll was half that and included Basij militiamen.
(Additional reporting by Anna Willard in Paris, editing by Mark Trevelyan)
