A nationwide rally on Monday to mark the killing of three students under the Shah turned violent when students clashed with security forces armed with batons and tear gas in the largest anti-government protests in months.
"From now on, we will show no mercy toward anyone who acts against national security. They will be confronted firmly," said prosecutor Gholamhossein Mohseni-Ejei, according to the official IRNA news agency.
Witnesses said scuffles occurred between students loyal to opposition leader Mirhossein Mousavi and others who support hardline President Ahmadinejad in Tehran universities on Tuesday.
Reuters could not confirm the report independently because journalists working for foreign media have been banned from leaving their offices from December 7 to December 9.
Mousavi's website, Kaleme, said security forces had a heavy presence around Tehran universities. It said Mousavi was harassed outside his office on Tuesday.
"If you want to beat me, threaten me or kill me, go ahead and do your job," Mousavi told a group of 30 masked, plainclothes men, Kaleme reported. "The men left after a few hours."
The protests were a renewed show of force following demonstrations that erupted after the June re-election of President Ahamdinejad, which the opposition says he won by rigging the vote.
Monday's protests in Tehran were smaller than the post- election rallies but the mood seemed more radical with protesters chanting slogans against the clerical establishment and not just criticizing Ahmadinejad's re-election.
Analysts say students have formed a bastion of support for opposition leader Mousavi.
"Silencing universities will be difficult for the establishment. Ahmadinejad's fate may well hang on them," said one analyst, who asked not to be named.
Iranian university students played a major role in toppling the U.S.-backed Shah Mohammed Reza Pahlavi 30 years ago and have always been a leading force behind political movements in Iran, both before and after the 1979 Islamic revolution.
"PEOPLE LEADING OPPOSITION"
A fierce crackdown on anti-government rallies, sweeping arrests of activists and harsh sentences imposed on leading reformists, including five death sentences, have so far failed to keep Iranians off the streets or quash the opposition.
"They feel they are being humiliated by the system. As long as the authorities continue to ignore their demands, protests will continue," said the analyst. "Now the people are leading the opposition leaders."
Dozens of people were arrested and several hurt in clashes in different Iranian cities on Monday. The opposition leaders did not attend the rally.
"About 200 demonstrators have been detained in Tehran. Some 39 of them are women," the semi-official ILNA quoted a senior police official as saying.
Also on Monday, the government banned the pro-reform Hayat-e No newspaper, a move which the New York-based Committee to Protect Journalists (CPJ) said was linked to the crack-down on demonstrations.
"Since the disputed presidential election in June, journalists have been censored, harassed and imprisoned. Iran now holds the dubious distinction of being second only to China as a jailer of journalists," the CPJ said in a statement.
When Ahmadinejad won the June election with a wide margin, his reformist opponents cried foul and thousands of Iranians took to the streets in the biggest anti-government demonstrations in the 30-year history of the Islamic Republic. Authorities deny any vote-rigging.
Prosecutor Mohseni-Ejei said the authorities had no intention of letting the demonstrations continue.
"Intelligence and security ... forces have been ordered not to give any leeway to those who break the law, act against national security and disturb public order," he said.
Iran's Revolutionary Guards and their allied Basij militia, which suppressed post-election unrest, have warned the opposition not to stage rallies against the establishment.
In September and November, opposition demonstrators clashed with government backers during rallies.
Mousavi criticized the clerical establishment on Sunday for suppressing students, saying the reform movement was alive despite pressure from the authorities to end it, his Kaleme website said.
Thousands were arrested after the election. Most of them have since been freed, but the judiciary continues to impose harsh sentences on arrested reformists, including former senior officials, lawyers, students and journalists.
In Geneva, United Nations High Commissioner for Human Rights Navi Pillay voiced concern on Tuesday that Iran is using more force to suppress protests and urged the Islamic republic to respect opposition supporters' right to protest.
"The suppression of protests is escalating, it is much more serious," Pillay told Reuters.
(Writing by Parisa Hafezi, Editing by Samia Nakhoul)