Tens of thousands of Iranians have taken to the streets of Tehran to mark the 31st anniversary of the Islamic revolution amid heavy security to prevent opposition protests.
Leaders of Iran's so-called Green Movement have called for its supporters to take part in Thursday's events, but police and pro-government militia were reported to be preventing many joining the rally in Tehran.
The son of Mehdi Karroubi, one of the defeated reformist candidates in last year's disputed presidential poll, told Al Jazeera that this father's car had been attacked by security forces on his way to the rally.
The opposition Jaras website reported that Karroubi was not seriously hurt when the windows of his car were smashed. Mohammed Khatami, a former president, has also reportedly been attacked.
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said: "Karroubi had announced where he would join the rally ... and people expected he would face some opposition along his way.
"The government really doesn't want any violence today, but it is not going to tolerate any kind of demonstrations with symbols of the opposition.
"We have heard from elsewhere in the city that opposition demonstrators who were trying to join the rally had been stopped on their way by security forces."
Khomeini's granddaughter Zahra Eshraghi and her husband, Khatami's brother Mohammad Reza Khatami, were briefly detained before being released after being told not to join the protests, sources told Al Jazeera.
Police said earlier on Thursday before the anniversary that a number of opposition figures had been arrested and security forces were ready to prevent any incidents.
"We are fully prepared for holding a safe and glorious rally," Esmail Ahmadi-Moghaddam, Iran's national police chief, 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."
State television footage showed a huge crowd at Tehran's Azadi square.
"From where I am standing I can see a sea of Iranians holding flags, waving placards, anti-Western placards, pictures of Ayatollah Khomeini, the former supreme leader who is viewed as the father of the revolution," Al Jazeera's Nazanine Moshiri, reporting from Azidi square, said.
"There is a heavy security presence all around me. There are also reports that in the last few days Basiji militia have been brought into the city."
"Speaking to opposition protesters in the last few days ... they have told me that they will be trying to attend the rally, they will becoming in a peaceful way."
Mahmoud Ahmadinejad, the Iranian president, made a speech to the crowd defending the country's right to further enrich its stockpile of uranium.
"By God's grace ... it was reported that the first consignment of 20 per cent enriched uranium was produced and was put at the disposal of the scientists," he said.
"In the near future we will treble its production."
Opposition websites have, in recent days, encouraged supporters of the Green Movement, which has staged a series of protests since the disputed election, to attend the rally, saying that the day belongs to all Iranians.
"I feel we have to participate [in the anniversary] while maintaining the collective spirit as well as our identity and leave an impression," Mir Hossein Mousavi, another of the defeated candidates in last year's presidential poll, said on Monday.
"Anger and bitterness should not take our control away."
Iran Green Voice, an opposition website, reported large numbers of opposition supporters gathering in several cities, including Tehran and the northern city of Tabriz.
The Rahesabz website said police had used teargas against crowds of opposition supporters in Tehran's Sadeghieh square.
However, it was impossible to independently verify the reports as foreign media have been banned from covering street marches.
Hundreds of thousands of Iranians gather every year to celebrate the overthrow of the ruling Shah in a revolution led by Ayatollah Ruhollah Khomeini.
Ali Akbar Hashemi Rafsanjani, the head of Iran's Expediency Council, touched on the tense political situation in an interview with state broadcaster IRIB.
"This year is more sensitive than 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 Mousavi and Karroubi of fraud and vote-rigging in the preisidential election.
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.