Security forces dispersed opposition protests as hundreds of thousands of government supporters massed on Thursday in a central square of the Iranian capital to mark the 31st anniversary of the revolution that created the Islamic republic.

Authorities clamped down hard to prevent a major show of force by the opposition amid one of the country’s most important political occasions. Tehran residents also reported Internet speeds dropping dramatically and e-mail services such as Gmail being blocked in a common government tactic to foil opposition attempts to organize.

Dozens of hard-liners with batons and pepper spray attacked the convoy of a senior opposition leader, Mahdi Karroubi, as he tried to join the anti-government protests, his son Hossein Karroubi told The Associated Press. The attackers - believed to be members of the Basij civilian militia - damaged several cars and smashed windows on Mr. Karrobi’s car, though he escaped unharmed, he said.

Security forces also briefly detained the granddaughter of Ayatollah Ruhollah Khomeini, the architect of the 1979 Islamic Revolution, and her husband, who are both senior pro-reform politicians, the opposition Web site Rahesabz reported.

It was one of the most high=profile detentions in the crackdown by authorities. The granddaughter, Zahra Eshraghi, and her husband Mohammad Reza Khatami, who is the brother of a former pro-reform president, were held for about an hour before being released, the site said. The report could not be independently confirmed, but the site has been credible in the past.

The anniversary 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 despite months of a fierce security crackdown.

In the revolution celebrations, 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, to attend a speech by Iranian President Mahmoud Ahmadinejad.

In Azadi Square itself, 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.

Mr. Ahmadinejad announced in his speech that the Islamic republic has produced its first package of highly enriched uranium just two days after beginning the process, proclaiming that Iran was now a “nuclear state.”

“The first package of 20 percent fuel was produced and provided to the scientists,” he said, without specifying 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 percent purity to 20 percent purity, bringing sharp criticism from the United States 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 percent plus level needed for a weapon - to fuel a research reactor for medical isotopes.

For days ahead of the anniversary celebrations, anti-government Web sites and blogs have called for a major turnout in counter-protests and urged marches to display green emblems or clothes, the signature colour of the opposition.

Opposition Web sites reported that protesters gathered in several places in Tehran on Thursday, displaying green banners and shouting slogans - but their numbers were not immediately clear. Witnesses said police deployed hundreds of forces in central Tehran to confront them.

Riot police fired paint-filled balls after several hundred protesters began to chant opposition slogans in Sadeqieh Square, about a half-mile (one kilometer) from the huge pro-government gathering, witnesses said.

Witnesses say there were no apparent injuries among the protesters.

The witnesses spoke on condition of anonymity for fear of retribution from authorities. Foreign media were only allowed to cover the ceremonies in the square and the speech by Mr. Ahmadinejad, with photographers bussed to the site and then away. There is an explicit ban on covering opposition protests.

Iranian authorities again tried to squeeze off text messaging and Web links in attempts to cripple protest organizers. Internet service was sharply slowed, mobile phone service widely cut and there were repeated disruptions in popular instant messaging services such as Google chat.

But several Iranians reached by The Associated Press said some messenger services, including Yahoo!, and mobile phone texting were still sporadically accessible. Many Internet users said they could not log into their Gmail account, Google’s e-mail service, since last week.

“We have heard from users in Iran that they are having trouble accessing Gmail,” Google said in a statement. “We can confirm a sharp drop in traffic and we have looked at our own networks and found that they are working properly.”

Opposition members went on roof tops late Wednesday and shouted Allah-u-Akbar (“God is greatest”. in protest - echoing similar cries after the disputed June election as well as anti-Shah protests more than three decades ago.

The opposition claims that Mr. Ahmadinejad’s victory in the June 12 election was fraudulent and that the true winner was pro-reform leader Mir Hossein Mousavi. Hundreds of thousands marched in the streets against the government in the weeks after the vote, prompting a massive wave of arrests.

Nevertheless, the opposition has succeeded in continuing to hold regular protests, often timing them to coincide with days of important political or religious significance in attempts to embarrass authorities. The tone of the rallies has shifted from outrage over alleged fraud in Mr. Ahmadinejad’s re-election to wider calls against the entire Islamic system, including Mr. Khamenei.

Tensions have mounted further since the last large-scale marches, in late December, which brought the most violent battles with security riots in months. At least eight people were killed in clashes between protesters and police, and security forces have intensified arrests in the weeks since.

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