Perhaps more importantly, they also saw an increased fervor and boldness among demonstrators, who more openly broke the biggest taboo in Iran -- burning pictures of Supreme Leader Ayatollah Ali Khamenei and chanting slogans against him.
The turnout, fueled by students marching by the thousands on more than a dozen campuses around the country, showed that months of arrests and government intimidation had failed to stamp out the movement, sparked by the disputed presidential election in June.
The fierce crackdown since the election has succeeded in limiting demonstrations, which have been held only about once a month at most. But authorities may be concerned because the coming months could heat up, with several key occasions that could give the opposition a pretext for bringing supporters into the street. The Islamic holy month of Moharram begins in mid-December, a time of frequent mourning ceremonies, and the anniversary of the 1979 Islamic Revolution will be marked over 10 days in early February.
On Tuesday, riot police were out in heavy numbers at intersections on major thoroughfares around the city.
At Tehran University, a group of hard-line Basij militiamen attacked a small demonstration of students Tuesday. The militiamen pelted them with stones and fired tear gas to disperse them and dragged at least one student away, said witnesses, who spoke on condition of anonymity fearing retribution.
Plainclothes men on motorcycles -- likely Basijis -- also harassed the opposition's leader, Mir Hossein Mousavi, at his Tehran office on Tuesday. Up to 30 men on motorcycles, some in masks, blocked Mousavi as he tried to drive out of his office garage and chanted slogans against him, two opposition Web sites said, citing witnesses.
Mousavi got out of his car and shouted at them, "You're agents, you've been tasked with threatening me, beating me, killing me," before his aides hustled him back inside, the Gooya News Web site reported. The men left several hours later and Mousavi was able to leave.
Hard-line clerics and commanders of the elite Revolutionary Guard have called for the arrest of Mousavi, accusing him of sparking protests and conspiring against Iran's clerical leadership. Arresting Mousavi or other top opposition leaders would be a major escalation, likely to spark greater turmoil -- and so far the government has balked at taking the step.
Tehran's police chief, Gen. Azizullah Rajabzadeh, announced that 204 protesters, including 39 women, were arrested in the capital during Monday's demonstrations. They were detained for "violating public order," including setting fire to vehicles and chanting slogans, he said, according to the state news agency IRNA.
Those arrested will be handed over to the judiciary for prosecution, he said. There was no immediate word on the number of arrests outside Tehran.
Iran's top prosecutor, Gholam Hossein Mohseni Ejehi, warned on Tuesday that the judiciary will be harsher than in the past. "So far, we have shown restraint. From today no leniency will be applied," Ejehi said, according to the official IRNA news agency.
He said Tehran prosecutors should take stronger action against those "who violate public order and damage public properties."
A wave of arrests since July succeeded in crushing the massive protests by hundreds of thousands that erupted after the election, which the opposition says President Mahmoud Ahmadinejad won by fraud.
More than 100 politicians, activists and protesters have been put on a mass trial before the Revolutionary Court, accused of being part of a foreign-backed plot to overthrow the Islamic Republic. Five have been sentenced to death and 80 others to prison terms up to 15 years. In recent weeks, more than 100 student leaders were arrested ahead of Monday's demonstration, and several have been given heavy prison sentences.
In recent months, the opposition has been focusing on holding protests timed to coincide with the many occasions in Iran's political and religious calendar that traditionally bring street demonstrations. The strategy aims to drum up as many people as possible and draw more attention.
Monday's protests were held on National Students Day, an annual day of marches commemorating the killing of three university students in 1953 during an anti-U.S. protest. The last major opposition protest was held in November on the anniversary of the 1979 seizure of the U.S. Embassy.
Ceremonies throughout the month of Moharram and the revolution anniversary celebrations will likely provide a similar opportunity for opposition marches.