Hundreds of thousands of government supporters today massed in central Tehran to mark the anniversary of the revolution that created Iran's Islamic republic - while president Mahmoud Ahmadinejad chose the day to proclaim his nation is now a "nuclear state".
A heavy security force fanned out across the city moved quickly to snuff out counter-protests by the opposition, clearing the way for the pro-government marches.
Police clashed with protesters in several sites around Tehran, firing tear gas to disperse them and paintballs to mark them for arrest, opposition websites reported.
Dozens of hard-liners with batons and pepper spray attacked the convoy of senior opposition leader Mahdi Karroubi, smashing his car windows and forcing him to turn back as he tried to join the protests, his son Hossein Karroubi said.
The celebrations were an opportunity for Iran's clerical regime to tout its power in the face of the opposition movement, which has persisted in holding mass street protests since disputed presidential elections in June in defiance of a fierce crackdown.
The security clampdown appeared to have succeeded in preventing a major opposition turnout. Their numbers were not immediately known, but opposition websites spoke of groups of protesters in the hundreds, compared to thousands in past demonstrations.
One protester, who did not want to be named, said she had tried to join the demonstrations but soon left in disappointment. "There were 300 of us, maximum 500. Against 10,000 people," she said.
"It means they won and we lost. They defeated us. They were able to gather so many people," she said. "But this doesn't mean we have been defeated for good. It's a defeat for now, today. We need time to regroup."
Foreign media in Iran were banned from covering the pro-reform protests. Tehran residents also reported internet speeds dropping dramatically and email services such as Gmail being blocked in a common government tactic to foil opposition attempts to organise.
Heavy numbers of riot police, members of the Revolutionary Guard and Basij militiamen deployed at key squares and major avenues in the capital to prevent protests from marring the annual mass rallies for the revolution's anniversary.
State television showed images of thousands upon thousands carrying often identical banners marching along the city's broad avenues toward the central Azadi, or Freedom, Square. There, the massive crowds waved Iranian flags and carried pictures of Ayatollah Ruhollah Khomeini, the founder of the Islamic state, and his successor as supreme leader Ayatollah Ali Khamenei.
In a nationally televised address in the square, Mr Ahmadinejad proclaimed that Iran has produced its first batch of uranium enriched to a higher level, saying his country will not be bullied by the West into curtailing its nuclear programme a day after the US imposed new sanctions.
"The first package of 20% fuel was produced and provided to the scientists," he said, reiterating that Iran was now a "nuclear state". He did not specify how much uranium had been enriched.
Iran announced on Tuesday that it was starting for the first time to further enrich uranium from around 3% purity to 20% purity, bringing sharp criticism from the US and its allies which accuse Tehran of seeking to develop a nuclear weapon.
Tehran, which denies seeking to build a bomb, has said it wants to further enrich the uranium - which is still substantially below the 90% plus level needed for a weapon - to fuel a research reactor for medical isotopes.