Iranian opposition groups are preparing to hold fresh protests against President Mahmoud Ahmadinejad on Monday, several websites reported, as the nation marks the annual Students Day.
The elite Revolutionary Guards and other authorities have warned they will crack down on any attempt by regime opponents to hijack the event to mount further demonstrations against Ahmadinejad's hotly disputed June re-election.
Every year on December 7, Tehran campuses mark the 1953 killing by the shah's security forces of three students, just months after a US-backed coup toppled the popular prime minister Mohammad Mossadeq.
Opposition protesters have already taken advantage of a number of regime-sponsored events to organise demonstrations on the streets.
On November 4, when Iran marks the anniversary of the 1979 seizure of the US embassy by radical students, thousands of protesters shouting "death to the dictator" clashed with security forces in central Tehran, who made more than 100 arrests.
On September 18, opposition protestors mixed with the massive crowds who marched through Tehran on the annual Quds Day display of solidarity with the Palestinians.
Several opposition websites have carried calls for new demonstrations near the main university campuses on Monday, although the two main opposition leaders -- Ahmadinejad's defeated challengers Mir Hossein Mousavi and Mehdi Karroubi -- have not joined the calls.
Petitions have been circulating on campuses calling on the two men to take part, student websites reported.
Opposition groups have warned anyone thinking of demonstrating on Monday that they are likely to meet a tough response from the security forces.
"Protesting in the streets on December 7 does not have the safety net of a state occasion... protesters are likely to face a violent crackdown," said leading reformist website Mowjcamp.com, which is close to Mousavi.
"The question is how the green supporters can have a strong presence with the least damage," it said, referring to the colour which was the symbol of Mousavi's campaign for the June election.
Police reinforcements have been deployed on the streets of Tehran in recent days and public prosecutor Gholam Hossein Mohseni Ejeie warned the regime is "determined to deal in a firm and deterrent manner with election protesters seeking to create unrest again."
Tehran police warned they would "confront any gathering or ceremony which is held outside the premises of universities."
"Such gatherings will be considered illegal," the police said in a statement carried by the ILNA news agency.
Brigadier General Ali Fazli, commander of the Revolutionary Guards in Tehran, said his force has "plans to tackle those who want to disrupt Students Day in the universities."
And cleric Ayatollah Ahmad Janati, who heads the powerful Guardians Council, used his sermon at the main weekly Muslim prayers on Friday to urge people to "refrain from doing anything that pleases the United States" on Monday.
Dozens of demonstrators were killed and thousands arrested in June when the regime cracked down on a wave of protests against the official election results giving Ahmadinejad victory.
Hundreds of thousands of people took to the streets of Tehran to demand the return of their "stolen votes," in the worst unrest since the 1979 revolution.