TEHRAN, Iran - Chants of protest began to be heard from rooftops in Iran's capital yesterday even as authorities choked off Internet access ahead of planned opposition demonstrations today.

Journalists working for foreign media were warned to stick to their offices for the next three days.

The measures were aimed at depriving the opposition of key means of mobilizing the masses as Iran's clerical rulers try to keep a tight lid on dissent.

Government opponents are seeking, nonetheless, to get large numbers of demonstrators to turn out today and show their movement still has momentum.

Opposition leader Mir Hossein Mousavi threw his support behind the planned student demonstrations and declared that his movement is still alive. A statement posted on his Web site said the clerical establishment was losing legitimacy in the Iranian people's minds.

"A great nation would not stay silent when some confiscate its vote," said Mousavi, who claims President Mahmoud Ahmadinejad stole the June 12 election from him.

Today's planned demonstrations mark the anniversary of the 1953 killing of three students at an anti-U.S. protest during the rule of Shah Mohammed Reza Pahlavi, a close American ally.

Since the 1990s, that anniversary has served as an occasion for protests by those urging Iran's Islamic leadership to allow more social and political freedoms.

Ayatollah Ali Khamenei, Iran's supreme leader, who has final say on state matters, accused the opposition yesterday of exposing divisions in the country and creating opportunities for Iran's enemies.

Iran's universities have been strongholds of the opposition movement that grew out of the disputed election, and authorities have besieged campuses with a wave of arrests and student expulsions.

The government's Basij militia has also recruited informers on campuses to blow the whistle on any opposition organizers, students said.

Despite heavy rain last night, rooftop cries of "Allahu Akbar" ("God is Great") and "Death to the Dictator" were heard from many parts of the capital Tehran.

The chanting reprised a tactic of the anti-shah movement in the 1979 Islamic Revolution and was revived after the recent disputed elections.

For weeks after the June presidential election, demonstrations triggered by claims of massive fraud in the vote brought hundreds of thousands to the streets, but the relentless crackdown that followed took a heavy toll.

Seeking to deny the protesters an organizing tool, authorities slowed Internet connections to a crawl, or shut it down, in Tehran.

The government has not acknowledged it is behind the outages, but Iran's Internet service providers said the problem was not a technical glitch.

Former President Akbar Hashemi Rafsanjani, who has been a powerful voice of dissent from within the ranks of the clerical leadership, accused Iran's hard-line rulers of silencing constructive criticism.

"The situation in the country is such that constructive criticism is not tolerated," Rafsanjani was quoted by several news agencies as saying yesterday.

Seeking to confine journalists working for international media to their offices during the protests, Iran's Culture Ministry suspended accreditation allowing them to report from the streets from today to Wednesday.