Tehran, Iran (CNN) -- Coming through on a promise to crack down on protesters on the 31st anniversary of the Islamic Revolution, Iran's security forces clashed with demonstrators Thursday, as hundreds of thousands filled a "disruption-free" Tehran square to hear their president announce the expansion of Iran's nuclear program.





Iran's celebration of the anniversary of the Islamic Revolution culminated February 11, a date that three decades ago marked the end of the country's Western-backed monarchy and the start of an Islamic republic. A coalition of Iranian reformist groups had urged opponents of Iran's hardline regime to stage nonviolent protests at central Tehran's Azadi Square on Thursday, on the official anniversary of the ouster of Iran's shah.





However, the Iranian government deployed thousands of police and plainclothes Basij militiamen in Tehran and other cities to quash anti-government protests during anniversary celebrations. Eyewitnesses said Iranian security forces quickly squelched out opposition protests, though CNN could not independently confirm the reports.





"The city was under siege. It was controlled entirely, it was impossible to protest the way people protested before," said Behzad Yaghmaian, author of "Social Change in Iran."





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"So the dangers were much higher today."





Witnesses said there was some confusion about organizing the protests; for example, it wasn't clear to some whether they were supposed to hold up their signs before or after they arrived at Azadi Square, where President Mahmoud Ahmadinejad was to speak.





It appeared that the strength of the opposition response was less than other recent days of unrest in Iran, namely the protests on the holy day of Ashura in late December. Because of reporting restrictions, there was no way to independently confirm the turnout of the opposition.





Yaghmaian said the so-called Green Movement, which is demanding democracy and fair elections, suffered a "numbers shock."





"The Green Movement anticipated a much larger turnout -- the turnout was not that large, and the turnout of the pro-government people was a lot larger than what people expected," said Yaghmaian, who teaches at New Jersey's Ramapo College.





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Plainclothed and uniformed security agents assaulted vehicles carrying reformist Mehdi Karrubi, who ran for in the disputed June presidential elections, and former President Mohammad Khatami as their supporters poured onto the streets, opposition sources said. Militia members also beat the wife of opposition leader Mir Hossein Moussavi with batons, according to postings on the social networking Web site Facebook and opposition Web sites.





The forces were preventing the opposition leaders and their followers from reaching Azadi, or Freedom Square, where Ahmadinejad delivered an anniversary address extolling the country's nuclear program to supporters. They fired on crowds in some areas and pepper-sprayed demonstrators in others, opposition groups said.





CNN has not been able to independently confirm those reports.





Members of the Basij, the paramilitary force loyal to Iran's hard-line leadership, attacked Karrubi while he was headed to a meeting with supporters, his son Mohammad-Taghi Karrubi told CNN. The militia broke a window in the car in which Karrubi was riding, an opposition Web site said. When he switched cars, that car also was attacked.





"The guards attacked, and the crowds came to him. When the crowds started to come and surrounded him, again the guards attacked with tear gas, tear as well as the batons and different kinds of weapons against the people. And unfortunately my father received very bad gas tears, and his face is burned," Mohammad-Taghi Karrubi said.





iReport video of opposition supporters in Tehran





The reformist Raheh Sabz Web site said plainclothes policemen arrested Karrubi's son Ali, as he tried to protect his father's car. Mohammad-Taghi Karrubi confirmed the arrest of his brother Ali, the third of the leader's four sons.





"My younger brother is arrested by police. And we were surprised because for two days we just came to demonstrate, to rally, to participate to show what we want and ask for our rights," he
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