Ayatollah Ali Khamenei thanked the "tens of millions" across the country who celebrated Thursday's anniversary, saying the turnout at rallies reflected the nation's strength.
During Thursday's anniversary celebration, security forces clamped down hard on scattered anti-government demonstrations in the Iranian capital.
Police clashed with opposition activists, firing tear gas to disperse them and paintballs to mark them for arrest. Groups of hard-liners also attacked senior opposition figures - including the wife of the head of the reform movement.
The massive government rally in central Tehran dwarfed the opposition gatherings, which were far smaller than other outpourings of dissent in recent months. Still, Thursday's events showed authorities must rely on full-scale pressures to keep a lid on demonstrations.
Khamenei said Thursday's rally should be a wake-up call for the "domestic enemies and deceived groups who claim to represent the people." He blasted the West, saying it was time for "foreign enemies to abandon futile efforts to subjugate" Iran.
"The past 31 years are not enough to awaken a few arrogant and bullying states to their futile efforts to dominate this Islamic nation," said Khamenei, who has final say on all state matters.
The authorities had worried ahead of the anniversary that any significant protests or clashes would be seen as a major embarrassment on a day intended to showcase national achievements and unity.
An array of riot police, undercover security agents and hard-line militiamen - some on motorcycles - had fanned out across Tehran on Thursday in what appeared to be the largest deployment since the post-election mayhem.
Hard-liners and security forces prevented opposition leader Mir Hossein Mousavi and his wife, Zahra Rahnavard, from attending an opposition gathering. Plainclothes Basij militiamen beat the 65-year-old Rahnavard with clubs on her head and back until her supporters formed a human ring around her and whisked her away, according to Mousavi's Web site.
Hard-liners also attacked the car of another opposition leader, Mahdi Karroubi, and smashed its windows.
Karroubi's son Taghi said his brother Ali, 36, was detained by security forces on Thursday and badly beaten. He was later released and the family took him to a hospital in serious condition, suffering from internal bleeding and a broken arm.
"He was severely tortured. They beat him to the verge of death," said Taghi Karroubi, speaking Friday on the phone to a reporter outside Iran.
However, Thursday's clashes were significantly less violent than previous opposition protest in late December, when eight people died and hundreds were arrested.
Authorities also jammed the Internet and mobile phones to disrupt the opposition. In Tehran, Internet speeds dropped dramatically and e-mail services such as Gmail were widely blocked.
Three major international broadcasters condemned Iran over its "deliberate electronic interference" in their broadcasts.
The BBC, Deutsche Welle and Voice of America said in a joint statement the jamming began Thursday. They said Iran was broadcasting freely around the world while denying its own people programs coming from the outside.
Iran's elite Revolutionary Guard, which was in charge of security for the rally, issued a statement Friday echoing Khamenei's words and saying the anniversary showed the will of the nation could not be defeated and is "strong as steel."
The government has regularly accused the U.S. and Britain of fomenting Iran's unrest. It has put more than 100 opposition activists and figures on trial since August on charges of fomenting the postelection turmoil. Ten have been sentenced to death and dozens to prison terms ranging from six months to 15 years.
In a nationally televised address from the anniversary gathering, President Mahmoud Ahmadinejad defied the West and boasted that Iran has become a "nuclear state." He dismissed new U.S. sanctions and denigrated President Barack Obama's efforts to repair relations.
Ahmadinejad said Iran has produced the first batch of 20 percent enriched uranium - sufficient strength to power Iran's research reactor - though he did not say how much uranium had been enriched.
Such a process has been at the heart of a U.N.-drafted proposal to provide Iran with reactor-ready fuel in exchange for its stockpile of low-enriched uranium. Iran has repeatedly blocked the plan with conditions and caveats.
The announcement of the higher-enriched uranium adds to Western worries that Iran has long-term goals to develop nuclear arms - even though it is still below the 90 percent-plus level needed for a weapon. Iran denies the accusation and insists it only seeks to produce energy and medical isotopes.
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Associated Press Writer Scheherezade Faramarzi in Beirut contributed to this report.
