TEHRAN, Iran - Hard-line militiamen firing tear gas and throwing stones stormed a crowd of thousands of university students protesting yesterday for a second day, as Iran threatened a tougher crackdown on the opposition after the biggest antigovernment demonstrations in months.

More than 200 people were arrested in Tehran on Monday during protests by tens of thousands at universities nationwide, and Iran's top prosecutor warned that further unrest would not be tolerated.

He hinted that authorities could even pursue the top opposition leader, Mir Hossein Mousavi, an escalation that the government has so far balked at amid Iran's postelection turmoil.

Masked motorcyclists - most likely hard-line militiamen - harassed Mousavi yesterday at his Tehran office. An angry Mousavi confronted them, daring them, "Kill me!" before being hustled away by aides, according to pro-opposition Web sites.

Authorities appear concerned that the protest movement could pick up new steam after Monday's demonstrations.

A fierce crackdown since the summer crushed the mass protests that erupted after June's disputed presidential election. But Monday's unrest showed how students have revitalized the movement. They showed increased boldness, openly breaking the biggest taboo in Iran, burning pictures of Iran's supreme leader, Ayatollah Ali Khamenei, and chanting slogans against him.

The protests spilled into yesterday. Several thousand students rallied at Tehran University, chanting slogans and waving Iranian flags in front of the Engineering College when they were assaulted by hard-line Basij militiamen, witnesses said. They said at least one student was dragged away.

Footage posted on the Internet, said to be from yesterday, showed the crowd of students sitting in front of the college building, many wearing surgical masks and scarves over their faces against gas or to hide their identities. They clapped and chanted "Death to the dictator" and insults against President Mahmoud Ahmadinejad and his supporters.

They are then seen fleeing as Basijis rush after them, firing tear gas and throwing stones. Students jostled in the crowd to get away, some crying out in warning, "Basiji, Basiji!" while women screamed, "God is great!" Inside the college building, students lit papers in a bonfire to try to ward off clouds of tear gas.

About a dozen students also clashed with riot police outside the university, said witnesses who spoke on condition of anonymity, fearing retribution. Foreign journalists have been barred from covering protests.

Tehran's police chief, Gen. Azizullah Rajabzadeh, said 204 protesters, including 39 women, were arrested in the capital Monday and would be handed over to the judiciary.

Iran's top prosecutor, Gholam Hossein Mohseni Ejehi, warned that the judiciary would no longer tolerate protests. "So far, we have shown restraint. From today, no leniency will be applied," he said, according to the official IRNA news agency.

When asked if the judiciary would pursue Mousavi, he said, "We will not tolerate anyone who commits actions against security, and we will confront them," according to the Fars news agency. He also suggested prosecutors could go after Mahdi Hashemi, son of former President Akbar Hashemi Rafsanjani, the most powerful supporter of the opposition in the clerical hierarchy.