Iranian security forces fired tear gas and used batons to disperse opposition protesters during rallies in Tehran to mark the 31st anniversary of the founding of the Islamic Republic.
Opposition Web sites say uniformed and plainclothes security officers patrolled central Tehran and moved quickly to halt anti-government protests. The opposition groups report at least 30 people were arrested.
The anti-government groups say leading reformist politicians Mehdi Karroubi and former President Mohammad Khatami were attacked at a rally.
They also report that security forces briefly detained Zahra Eshraghi, the grand-daughter of the leader of the Islamic Revolution, the late Ayatollah Ruhollah Khomeini. Eshraghi was held along with her husband, Mohammad Reza Khatami, who is the brother of the Mr. Khatami.
The opposition reports could not be independently verified because Iran has banned foreign press coverage.
President Mahmoud Ahmadinejad addressed hundreds of thousands of people who gathered in Tehran's Azadi Square to celebrate the national holiday.
In his address, Mr. Ahmadinejad praised the achievements of the Iranian people, criticized the West, and announced that Iranian scientists had produced a first batch of higher-grade nuclear fuel.
Opposition leaders Mir Hossein Mousavi and Mehdi Karroubi had urged their supporters to turn out for a peaceful demonstration Thursday.
The anniversary marks the 1979 ouster of Iran's monarchist government. Every year, hundreds of thousands of Iranians take part in rallies to celebrate the anniversary.
Mass anti-government protests broke out after Iran's disputed presidential election in June. Opposition leaders have accused President Ahmadinejad of stealing the vote.
The Middle East director at U.S.-based Human Rights Watch, Joe Stork, says the Iranian government is trying to use anniversary celebration to "deflect attention" from its human rights violations.
The rights group has released a new report that claims to show the Iranian government's crackdown on dissent since the election has been worse than previously reported. The report documents cases of extra-judicial killings, rapes and torture, and other serious rights violations.
There also are reports that Iranian authorities have slowed down Internet service in the country and have blocked Google's e-mail service, Gmail.
The California-based Internet company confirmed a sharp drop in e-mail traffic, and acknowledged that users in Iran are having trouble accessing Gmail.
In a statement, Google said it believes "people everywhere should have the ability to communicate freely online," adding, "sadly, sometimes it is not within our control."