Leaders of Iran's opposition movement have called for demonstrators to take to the streets as the Islamic republic marks its 31st anniversary.
Police, Revolutionary Guard troops and Basij militia have been deployed in force for the anniversary, which is marked annually with huge displays of support for the 1979 revolution.
A number of opposition figures were detained before the event on Thursday, police said.
Esmail Ahmadi-Moghaddam, Iran's national police chief, said that the Revolutionary Guard and Basij militia were ready for any incident.
"We are fully prepared for holding a safe and glorious rally," he told the semi-official Fars news agency.
"We are closely watching the activities of the sedition movement and several people who were preparing to disrupt the February 11 rallies were arrested."
Mahmoud Ahmadinejad, the Iranian president who was re-elected in a disputed poll last year, is set to address a mass rally in Tehran's central Azadi square.
"I have no doubt that Bahman 22 [February 11] of this year will make clear the right path and will cause the world to come to a major decision," Ahmadinejad said in televised speech ahead of the anniversary.
"And with the grace of God it [the rallies on this day] will create a greater victory and honour for the Iranian nation."
In previous years the event has attracted tens of thousands of Iranians celebrating the overthrow of the ruling Shah in a revolution led by Ayatollah Ruhollah Khomeini.
Ahmadinejad made no remarks about possible protests on the anniversary or the previous unrest in the country.
Opposition websites have, in recent days, encouraged supporters of the so-called Green Movement, which has staged a series of protests since the disputed election, to attend the rally as well, saying that the day belongs to all Iranians.
Ali Akbar Hashemi Rafsanjani, the head of Iran's Expediency Council, touched on the tense political situation in an interview with state broadcaster IRIB.
"I feel we have to participate while maintaining the collective spirit as well as our identity and leave an impression"
Mir Hossein Mousavi,
"This year is more sensitive that previous years. First because of the internal disputes which enemies have focused on," the former president and bitter rival of Ahmadinejad said.
The opposition has used previous state-backed events, such as the annual al-Quds Day in September, to denounce Ahmadinejad and his government.
Scores of people have been killed in clashes between security forces and the protesters over the past eight months and hundreds of people have been arrested.
The demonstrations were sparked by accusations by Mir Hussein Mousavi and Mehdi Karroubi, two defeated reformist candidates in the presidential election, of fraud and vote-rigging in the preisidential election.
"I feel we have to participate [in the anniversary] while maintaining the collective spirit as well as our identity and leave an impression," Mousavi said on Monday.
"Anger and bitterness should not take our control away."
Karroubi said: "Let us all together take part in the anniversary rally calmly and firmly ... with patience and without verbal and physical violence."
Internet connections reportedly slowed to a crawl before the anniversary and text messaging services were disrupted, with the government blaming technical glitches.
The opposition has previously used the internet and SMS messaging to organise protests and spread news and pictures of events.
The anniversary takes place with Iran under pressure from the United States over its decision to further enrich some of its uranium stockpile, which Tehran says is for use in a medical research reactor.
The US treasury announced sanctionsagainst a Revolutionary Guard commander and four companies on Wednesday over Iran's nuclear programme, which Washington says is a cover for a weapons programme.