Iranian opposition websites said security forces clashed with protesters and arrested at least 30 as huge crowds flocked to central Tehran on Thursday to mark the 31st anniversary of the Islamic revolution.
Official media made no mention of clashes or arrests and state television said "tens of millions of people" attended rallies in support of the revolution across the country of 70 million.
The Islamic state is facing its worst domestic crisis in three decades as opposition supporters have rallied round the reformists who lost to President Mahmoud Ahmadinejad in a disputed election last June.
Ahmadinejad told a vast, flag-waving crowd of government supporters that Iran was now able to enrich uranium to more than 80 percent purity, coming close to levels experts say would be needed for a nuclear bomb, although he again denied it had any such intention.
"The Iranian nation is brave enough that if one day we wanted to build nuclear bombs we would announce it publicly without being afraid of you," Ahmadinejad said, addressing Iran's Western enemies.
But he told the crowd in Azadi (Freedom) Square: "When we say that we don't build nuclear bombs, it means that we won't do that because we don't believe in having it."
State television showed live footage of hundreds of thousands of people, some carrying Iranian flags and pictures of Supreme Leader Ayatollah Ali Khamenei, walking to the square.
An opposition website, Iran's Green Voice, said security forces fired shots and teargas at supporters of opposition leader Mirhossein Mousavi staging a rally in central Tehran. Mousavi and his wife attended one of the rallies, it said. Another opposition site, Norooz, said 30 people were arrested in one Tehran square. A third, Jaras, said security forces attacked another opposition leader, Mehdi Karoubi, and moderate former president Mohammad Khatami. It said the windows of Karoubi's car were smashed but he was not seriously hurt.
The reports could not be verified independently because journalists working for foreign media were escorted to Azadi Square and are not at liberty to cover opposition rallies.
There were no reports of the kind of violence that erupted in late December, when eight people were killed in clashes between security forces and opposition supporters.
Neither side has shown much appetite for compromise in the eight months since the presidential vote, which the opposition says was rigged to secure Ahmadinejad's re-election. The authorities insist it was fair.
Since June, thousands of people protesting against the conduct of the vote have been arrested. Most have since been freed, though more than 80 people have been jailed for up to 15 years, including several senior ex-officials.
In January, Iran hanged two people sentenced to death in post-vote trials and at least nine others are appealing such sentences.
The country faces growing Western calls for a new round of targeted United Nations sanctions against it after Ahmadinejad this week ordered a start to production of higher-grade uranium.
Iran says it moved to produce the 20 percent enriched uranium for a Tehran research reactor making medical isotopes out of frustration at the failure to reach agreement on a uranium exchange with world powers.
"By God's grace ... it was reported that the first consignment of 20 percent enriched uranium was produced and was put at the disposal of the scientists," Ahmadinejad said. "In the near future we will treble its production."
The president said Iran has "the capability to enrich uranium to much higher levels" at its Natanz enrichment plant.
Iran had previously purified the fuel to just 3.5 percent, the level required for a nuclear power plant.
Western experts say the jump to 20 percent is a major technical leap towards enriching uranium to the 90 percent-plus that would be needed for a nuclear bomb.
The West accuses Iran of covertly trying to build nuclear bombs. Iran, the world's fifth-largest crude oil exporter, says its nuclear facilities are part of a peaceful energy programme and has said it would retaliate for any attack on them.