Iranian police say they have arrested a number of opposition supporters planning demonstrations during the anniversary of the Islamic Revolution.
Police chief Esmail Ahmadi Moghaddam did not give any details of the arrests, the Fars news agency said.
Opposition leaders have called on supporters to gather on Thursday, raising the possibility of violence in cities across the country.
The authorities have said they will crush any anti-government protests.
There have been a number of outbreaks of violence since disputed elections in June 2009, scores of people have been killed and scores more imprisoned, government opponents say.
Until last year, public protests against Iran’s leadership were virtually unheard of.
But after President Mahmoud Ahmadinejad was re-elected last June, opposition leaders have been calling supporters out onto the streets.
The Iranian leadership has countered with threats, saying a display of unity from Iranians will deliver a “punch in the face” to opposition reformists and the West, which they accuse of fostering dissent.