Tehran, Iran (CNN) -- Security forces clashed with demonstrators Thursday as Iran marked the 31st anniversary of the Islamic Revolution, while thousands blanketed a Tehran square to hear their president announce the expansion of Iran's nuclear program.

Pro-government security personnel -- both plainclothes and uniformed -- assaulted vehicles carrying Mehdi Karrubi, a reformist leader who ran for president in the disputed June presidential elections, and former President Mohammad Khatami as their opposition 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 opposition leaders and their followers -- the so-called Green Movement -- from reaching Azadi Square (Freedom Square) in central Tehran, where President Mahmoud 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 opposition 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.

The 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 Ali Karrubi, who is 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 said.

Opposition Web sites said plainclothes security forces wielding tear gas and batons also attacked Khatami's vehicle and supporters as he headed to Azadi Square, forcing him to abort a plan to walk to the square with followers, Raheh Sabz said. Two reformist figures, Mohammad Reza Khatami -- the brother of the former president with a similar name -- and his wife, Zahra Eshraghi, were arrested, opposition groups also said.

The Facebook page of Dr. Zahra Rahnavard, Moussavi's wife, said she had planned to join demonstrators at Sadeghiye Square, when she "was surrounded and attacked by plainclothes militia.

"Plainclothes militia physically assaulted her and beat her with batons at her head and back. Zahra Rahnavard after this incident with the support of a large crowd of people who made a human shield to protect her, was able to leave the area," the Facebook page said.

Iran's Press TV took note of opposition protests, saying "a few hundred supporters" of Moussavi and Karrubi gathered in a western Tehran district, and Karrubi was seen among the demonstrators.

"Police had stepped up security in the area to prevent possible disturbances," it reported.

Observers say the pro-government Azadi Square demonstration was a well-choreographed event.

One observer in the throng told CNN a relaxed and supportive atmosphere prevailed at the demonstration, where there was no sign of Basij militia members, ever-present during opposition events, but they were seen stationed away from the main area of the demonstration.

Government officials distributed cartons of milk, juice boxes, lemonade, and cupcakes in the crowds, which appeared to number in the hundreds of thousands, the witness said.

Women who usually stay home went out, and a number of people not from Tehran were there, the observer said.

"They got what they wanted, a disruption-free rally," the observer said. "There were no potential 'greens' in that crowd.

Flag-waving and cheering supporters heard Ahmadinejad announce that Iran has produced its first batch of 20-percent-enriched uranium and will soon triple production.

Uranium enriched to 20 percent can set off a nuclear reaction, scientists say, and the president said the country is capable of enriching uranium up to 80 percent, but won't.

White House spokesman Robert Gibbs told reporters Wednesday that "We do not believe they have the capability to enrich to the degree they say they are enriching."

However, the West has long been alarmed over what it believes is Iran's intention to build nuclear weapons. But Tehran insists it is not building nuclear weaponry, and wants to use the technology for civilian purposes, such as supplying medical isotopes for cancer patients.

"For a long time they said you can't have this or that. You have to shut down this facility or that," Ahmadinejad said. "[Western nations said Iran] should basically throw away all of our facilities but again, our nation stood firm, our dear leadership stood firm and with the grace of the 12th Imam of the Shias, our nation today was victorious in the end and the Iranian nation became a nuclear nation."

He also touched on familiar topics: He lashed out at the West, particularly the United States, and criticized its relationship with Iran's rival, Israel.

Ahmadinejad's speech at Azadi Square ended without incident.

"There are those who stand against us because what they want is to control the Middle East and to control the world," Ahmadinejad said. "And an independent Iran, a free Iran, a developed Iran, a powerful Iran will naturally be a challenge for their goal. And that is why they are hostile to the Iranian nation."

Since a disputed presidential election in June, anti-government protesters have turned public gatherings into rallies against hard-liner Ahmadinejad, who was declared the overwhelming winner of the race, and street demonstrations were anticipated on the anniversary of the Islamic Revolution.

Over the volatile months since June, police have responded to such demonstrations with mass arrests, denouncing protesters as anti-Islamic and against the revolution. In the latest big street confrontation, on the holy day of Ashura in December, witnesses said hundreds of people were detained and seven were killed, with responsibility for the fatalities denied by the Iranian regime.

As the anniversary day approached, opposition leaders urged their followers to demonstrate peacefully, and Iran has imposed tight restrictions on foreign reporters covering the anniversary, busing them to and from Ahmadinejad's speech to prevent them from reporting on skirmishes on the streets.

The precautions were meant to prevent a repeat of overt anti-government displays on other key occasions that have embarrassed and inflamed Iranian authorities.

Pickup trucks roamed the streets of Tehran, blaring pro-government slogans and songs from speakers, a witness said, and residents in the Iranian capital said Wednesday that text messages on many messaging services have been blocked and Internet speeds have slowed to a crawl.

At Aria-Shahr square in western Tehran and in various other parts of the city, security forces fired on and tear-gassed demonstrators who chanted "Death to the dictator," and "Death to [Supreme Leader Ayatollah] Khamenei," according to opposition reports. Undeterred, the demonstrators chanted on, and many of them wore masks or handkerchiefs over their faces.

Opposition videos surfacing from Tehran showed the wrath of demonstrators. One showed protesters tearing up a poster of Ayatollah Khomeini, the late founder of the Islamic Revolution, and Ayatollah Ali Khamenei, Iran's supreme leader. Another showed people stomping on an image of Khamenei. One video showed a man apparently stripped from the waist up put in a choke hold and punched in the head by a helmeted security force member.

Follow CNNIranDesk on twitter

The post-election crackdown on dissidents in Iran -- which has led to executions and jailings -- has been condemned throughout the world.

Catherine Ashton, the European Union's foreign policy chief, said the EU is concerned over "reports that opposition leaders have been subjected to violence and intimidation" and that some may have been detained.

She said, "The scenes of violent repression today are part of a pattern over the past few months."

She said the EU supports the protesters, whose "determination demonstrates the strength of their desire" for freedom and democracy.

"The choices this regime is making vis-à-vis its people and the international community are the wrong ones. No amount of inflated rhetoric can hide that. The regime is letting its own people down; and they deserve better."

The government said that if protesters disrupt state-sanctioned marches, they will be jailed until at least April 9, the end of the Persian holiday of Norooz. The holiday marks the start of spring.

CNN's Reza Sayah contributed to this report