TEHRAN: Iranian opposition leaders Mohammad Khatami and Mehdi Karroubi came under attack and their supporters clashed with police as vast crowds massed in Tehran on Thursday to mark the 1979 Islamic revolution.

The opposition's ability to mount protests despite a massive security force deployment is seen as highly symbolic given the anniversary's historic significance.

The elite Revolutionary Guards and police had warned they would crack down heavily on any protests which, since they first erupted last June, have threatened the very pillars of the Islamic regime and split the senior clergy.

Opposition website Rahesabz and witnesses said the clashes took place at Sadeghieh square, about a kilometre (mile) from where Iranians had gathered from early morning at Azadi (Freedom) Square, to mark the toppling of the US-based shah 31 years ago.

The website said the cars of the two leaders came under attack by police and plainclothes security men but neither was hurt. It added that police had fired teargas at crowds of opposition supporters at Sadeghieh square.

The reports could not independently be confirmed as the foreign media has been barred from covering the street marches.

Rahesabz also reported that ex-president Khatami's brother Mohammad Reza and his wife Zahra Eshraghi, the granddaughter of the Islamic Revolution founder Ayatollah Ruhollah Khomeini, were arrested by security forces but later released.

Karroubi's son Hossein told AFP that his father was "not injured but his guards who were accompanying him were."

"They (police and plainclothes men) fired tear gas and were brandishing knives when they clashed with our supporters" before the cleric reached Sadeghieh square in western Tehran from where he was supposed to join the marches.

Karroubi's other son Ali was also arrested, Hossein said.

State television showed footage of men, chador-clad women and children carrying banners reading "Death to America, Death to Israel!" massed at the Azadi (Freedom) Square celebrating the anniversary.

President Mahmoud Ahmadinejad in an address to the crowd said Iran has produced a "first stock" of 20 percent enriched uranium for its nuclear programme and is capable of enriching it to 80 percent but will not do so.

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The hardline president also said Iran would soon triple its daily production of low-enriched uranium (3.5 percent) and lashed out at US President Barack Obama, saying his American counterpart was "missing opportunities" and serving the interests of Israel.

"They (Americans) want to dominate our region but the Iranian people will never let them do that," he said to roars from the crowd, adding that the "Iran nation has now become a nuclear" state.

Celebrations to mark the anniversary have traditionally been festive, and an opportunity for Iranian leaders to showcase popular support for the establishment.

But this year's event was disrupted in part by anti-government protests, with Rahesabz reporting that "very large crowds" had gathered at Sadeghieh square.

A witness told AFP that "police also fired tear gas and several rounds from air guns at opposition supporters."

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He said people who had gathered at the square were also "hit with batons and iron rods by plainclothes men."

Hitting back at official efforts to stifle news of opposition protests, the opposition launched an impromptu radio station on the Internet late morning.

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The scratchy, live broadcast flashed news reports about the attacks on opposition leaders, and clashes between protesters and security forces, including the Basij militia.

The radio reported many clashes around the capital, including large confrontations with security forces at Vanak Square, several kilometres from Azadi square. It said there was a large number of arrests.

Ahmadinejad's disputed re-election last June plunged the Islamic republic into one of its worst ever political crises, with the opposition refusing to take the fight off the streets despite often deadly crackdowns.

Most recently, eight people were killed on the Shiite holy day of Ashura on December 27 and hundreds were jailed as authorities battled protesters they accuse of seeking to topple the regime and siding with Iran's enemies abroad.

Iran's all-powerful supreme leader Ayatollah Ali Khamenei said he wanted Thursday's celebration to be a show of unity and to deliver a stunning "punch" to "arrogant" powers.

The opposition is led by former stalwarts of the Islamic republic, including one-time premier Mir Hossein Mousavi, who says the 1979 revolution failed because the shah-era "roots of tyranny and dictatorship" still exist.

Ahead of the anniversary, Internet connections slowed to a crawl and text messaging services were disrupted, with the government blaming technical glitches. Google said Iranian users are having trouble accessing Gmail.