The demonstrations sparked by social media posts and a surge in prices of basic food supplies are the largest since the protests that followed the country's disputed 2009 presidential election.
A wave of spontaneous protests over Iran's weak economy swept into Tehran on Saturday, with college students and others chanting against the government.
This happened just hours after hard-liners held their own rally in support of the Islamic Republic's clerical establishment.
The demonstrations appear to be the largest to strike Iran since the protests that followed the country's disputed 2009 presidential election.
Thousands already have taken to the streets of cities across Iran, beginning at first on Thursday in Mashhad, the country's second-largest city and a holy site for Shiite pilgrims.
The protests appear sparked by social media posts and a surge in prices of basic food supplies, like eggs and poultry.
After a media blackout in first two days of protests, officials and state media made a point on Saturday of saying Iranians have the right to protest and have their voices heard on social issues.
There was chaos around the Tehran University as several hundred people scuffled with police and shouted slogans against the regime for several hours, bringing traffic to a standstill.
But the regime also put on a show of strength, with hundreds of counter-demonstrators seizing control of the university entrance in Tehran, chanting "Death to the seditionists".
Protests on Friday also struck Qom, a city that is the world's foremost center for Shiite Islamic scholarship and home to a major Shiite shrine.
At least 50 protesters have been arrested since Thursday, authorities said.
Earlier Saturday, hard-liners rallied across the country to support Supreme Leader Ayatollah Ali Khamenei and others.
The authorities were fortunate that annual rallies marking the defeat of the last major protest movement in 2009 were already scheduled for Saturday morning and brought thousands of regime enthusiasts to the streets across the country.
There has been particular anger at welfare cuts and fuel price increases in the latest budget announced earlier this month.
Since the 2009 protests were ruthlessly put down by the Revolutionary Guards, many middle-class Iranians have abandoned hope of pressing for change from the streets.
But low-level strikes and demonstrations have continued, often on a sector-by-sector basis as bus drivers or teachers or workers from specific factories protest against unpaid wages or poor conditions.
Some of this week's protests were directed against financial scandals linked to unauthorised lending institutions which collapsed with the loss of hundreds of thousands of accounts.
Unemployment remains high. Official inflation has crept up to 10 percent.