WATCH: Students at Amir Kabir University hold up money to taunt plainclothes members of the Basij militia, chanting, "We are not like those who get money to stand [and supress]."
Iranian police have fired tear gas and warning shots and used batons to disperse opposition protesters chanting antigovernment slogans in central Tehran, RFE/RL's Radio Farda reports.
Critics of the country's disputed presidential election had vowed to use today's commemoration of Students Day, when officials traditionally stage anti-Western public displays, to show that Iran's reform movement is still strong.
There was also fighting between government critics and supporters.
Witnesses say opposition acts of defiance began overnight, when supporters braved pouring rain to climb to Tehran rooftops and shout "Allahu Akbar" and "Death to the dictator."
Tough Tactics
Iranian riot police cordoned off Tehran University -- Iran's largest university and the scene of student-led, pro-democracy protests in 1999 -- in an apparent effort to prevent opposition plans to disrupt a state-organized rally. Hundreds of security officers and antiriot motorcycle units had been deployed outside the campus.
A crowd at Amirkabir Technological University, where pro- and antigovernment protesters were on hand.
"Police and security forces aren't allowing people to get close to the university and people can't even walk in the pedestrian area around the campus," one man told Radio Farda from the scene. 
He described "a very big clash between riot police and people" as the authorities used tear gas to disperse a crowd on Jomhuri Street, south of Tehran University.
The eyewitness added that security forces were videotaping the events, possibly to help identify participants for subsequent prosecution.
Reuters quoted an opposition website as saying that at least two women were arrested outside Tehran University today. Both were reportedly wearing green ribbons to symbolize support for the opposition.
Later eyewitness reports suggested more arrests, but those claims could not be confirmed.
The same reformist website claimed the main mobile-telephone network in Tehran and at the university had been shut down, according to Reuters.
There are reports of other gatherings, including at Tehran's Amirkabir Technological University, a hotbed of antigovernment protest, where officially organized groups are competing for space with opposition supporters.
"Police are not allowing people to get together, and Basijis and police are attacking people [in Vali Asr Square]," a Tehrani woman told Radio Farda. "At the moment, I am close to the main entrance door of Amir Kabir University, and people have crashed the main entrance door and are getting onto the campus."
An Amir Kabir student website was quoted as saying that authorities were preventing people from leaving the Tehran University campus in order to keep them from joining protests elsewhere.
Radio Farda has confirmed that protests also took place in the cities of Mashhad, Esfehan, Kerman , Shiraz and Kermanshah. There were unconfirmed reports that protests also took place at universities in Arak, Tabriz, Shahre Kord, and Hamedan.
Reform Hopes
The authorities on December 5 banned foreign media from reporting on Student Day events -- saying they may not leave their offices between December 7 and 9 -- in their latest attempt to avert fallout from the divisive June election that they called for hard-line President Mahmud Ahmadinejad.
Mir Hossein Musavi has challenged the authorities' plans to suppress rallies and subsequent protests.
The opposition led by former Prime Minister Mir Hossein Musavi has rejected that vote as fraudulent. 
In a statement on his website, Musavi indicated the opposition protests would continue indefinitely. He said that even if the authorities managed to quell the opposition rallies today, there was still tomorrow, and the day after tomorrow.
The clampdown by Iran's clerically led establishment has included mass arrests, reports of torture, and disappearances. But after all these pressures, the movement has not ended, Musavi said.
Meanwhile, another reformist presidential candidate, Mehdi Karrubi, suggested in "Le Monde" today that it's too soon for a government-opposition rapprochement. He said that "restoring confidence between the people and authorities" was necessary, but that "some fundamentalists, the more moderate ones, share this view but unfortunately they don't have a lot of power."
Making Life Difficult
Police and the Islamic Revolutionary Guards Corps had warned that any "illegal" rallies will be fiercely confronted.
A snapshot of riot police guarding Tehran University this morning, in anticipation of opposition protests.
Internet speeds had slowed dramatically over the past day in an apparent effort to hamper opposition communication.
The Associated Press has quoted residents of the capital as saying they were unable to access e-mail, and that the government's blockage of opposition websites has been tightened.
Iran's clerically dominated authorities traditionally organize large rallies on Students Day to condemn the United States and other alleged Western interlopers.
In Washington, White House officials said they were "concerned with the crackdown."
State Department spokesman Ian Kelly said, "We're disturbed any time we see people who are trying to exercise their peaceful democratic rights being prevented from doing so by means of cutting off their access to information, cutting off their ability to communicate their views, and by arbitrary arrest and detention."
He added, "We have concerns about Iran and its treatment of its own people. Iran says that there needs to be more justice in international affairs. Well, they have to show justice at home before we can take that particular statement seriously."
written in Prague from Radio Farda and wire reports