People have continued to come out on the streets despite threats
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.
RECENT UNREST IN IRAN
19 Dec: Influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri dies aged 87
21 Dec: Tens of thousands attend his funeral in Qom; reports of clashes between opposition supporters and security forces
22 Dec: Further confrontations reported in Qom
23 Dec: More clashes reported in city of Isfahan as memorial is held
24 Dec: Iran reportedly bans further memorial services for Montazeri except in his birthplace and Qom
26 Dec: Clashes reported in central and northern Tehran
27 Dec: At least eight dead following anti-government protests in Tehran; 300 reported arrested
Mehdi Karoubi, one of the defeated contenders for the presidency, has said he will attend a rally in Tehran, and that his supporters should "follow up on their legal demands silently, but strongly as before".
He said people should not provoke government forces by chanting radical slogans.
The pro-government Basij Islamic militia have been charged by the authorities with keeping order during Thursday's anniversary of the 1979 revolution that toppled Iran's royal family.
The militia, who have been accused of shocking violence during past demonstrations, are "ready for any possible incident", Mr Moghaddam was quoted by the Reuters news Agency as saying.
"They will let no-one create insecurity," he said.
In December, eight people were killed in clashes at demonstrations during Ashura, one of the holiest days in the Shia Muslim calendar.
At least 30 protesters have been killed in clashes since the 2009 election, although the opposition says more than 70 have died.
More than 80 people have been jailed for up to 15 years, including former government spokesman Abdullah Ramezanzadeh, former vice-president Mohammad Ali Abtahi, former deputy economy minister Mohsen Safaie Farahani and former industry minister Behzad Nabavi.
Some 200 activists remain in detention.