Street fights broke out between youths and Basij militiamen, supporters of the Islamic republic, according to eyewitnesses. Police fired tear gas at groups of youth near Tehran University to disperse clashes.
Police also clashed with Basij volunteers, who were trying to approach an area where a traffic jam of protesters’ cars was backed up, in an effort to prevent further violence.
Witnesses at the scene described Tehran as tense.
There were also conflicting reports over the fate of three Iranian student leaders.
According to some reports, they were arrested earlier in the day minutes after holding a news conference in which they planned to criticise authorities for preventing them from holding protests.
But other witnesses said a group of vigilantes had seized the three, who were holding a conference to announce the cancellation of planned demonstrations.
Iranian authorities had banned gatherings and closed campuses in the run-up to the anniversary of the 1999 student unrest, when vigilantes attacked a Tehran university dormitory. One person was killed, sparking five days of mass protests.
In June, Iranians also took to the streets for a series of 10 nights, protesting against the government.
The United States strongly supported the demonstrations, which picked up momentum after a US-based Iranian opposition television station called on ordinary Iranians to join protesting students.