Iranian authorities have warned opposition supporters against turning the country's annual al-Quds (Jerusalem) Day Palestinian solidarity rally into a protest against the Iranian government.
Opposition activists have called for mass protests to coincide with the government-sponsored demonstration on Friday marking Iran's annual show of solidarity with the Palestinians.
But Iran's Revolutionary Guard warned on Thursday that it would crack down to halt any anti-government rallies.
"This nation's brave children who are in the security bodies and the police, or in the Revolutionary Guards or the Basij (are ready) to confront firmly any deviation, and anti-revolutionary ... moves," the Guards said in a statement published by the state news agency, Irna.
Ayatollah Ali Khamenei, Iran's supreme leader, has also warned the opposition against using the al-Quds Day rally to launch street protests.
"Be watchful so some who want to spread division do not succeed. No division should be created," he said recently.
Ayatollah Ruhollah Khomeini, the late founder of Iran's 1979 Islamic revolution, declared the last Friday of the Muslim holy month of Ramadan as al-Quds Day, calling for international rallies in support of Palestinians and against Israel.
Reformist websites have said opposition leaders would attend the rallies on Friday and they called on supporters to raise anti-government slogans during the gatherings.
Mahmoud Ahmadinejad, Iran's president, who is accused by opposition groups of rigging his re-election in June, is expected to mark the day with an address at a rally at Tehran University.
His main rivals, defeated presidential candidates Mir Hossein Mousavi and Mehdi Karroubi, have also said they will take part in the main al-Quds Day march.
'Regret' for deaths
The two opposition candidates led mass protests in the wake of the elections, sparking a major crackdown by police and the detention of hundreds of people.
The opposition says at least 69 people were killed in the crackdown, including some who died from torture in prison.
The June election returned Ahmadinejad to power, but the opposition has disputed the poll, saying it was rigged.
Ahmadinejad told US television network NBC News that he regretted the deaths of protesters during the demonstrations.
"All of us regret the fact that some people were killed," Ahmadinejad said in excerpts of an interview to be aired on Sunday.
But he said he did not "see any problems" with the credibility of the June 12 election.
More than 100 prominent opposition supporters are now on trial on accusations of plotting to use the anti-election protests to overthrow Iran's clerical leadership.