Violence had marked previous anniversaries
Disturbances have taken place in Tehran, as protesters marked the anniversary of a raid on a student dormitory.
Thousands of motorists, many with their families, converged on the city centre near Tehran University after nightfall, sounding their horns and forming huge traffic jams.
The whole area was swamped by security forces, including riot police, regular police, plainclothes men and hundreds of hardline vigilantes on motorbikes, says the BBC's Jim Muir in Tehran.
The Iranian authorities had taken stringent measures to try to head off trouble on the anniversary following 10 days of late-night protests against the Islamic regime last month.
Wednesday's protest marked the fourth anniversary of a raid on a student dormitory in 1999 by police and right-wing vigilantes, which triggered several days of violent street riots.
On Tuesday, there were occasional clashes and other incidents and a number of people were arrested, but there seemed to be no focal point for the trouble and no real crowd as such since most people were in their cars, our correspondent says.
There were reports that tear gas was fired to disperse protesters.
One report said official security forces clashed with right-wing vigilantes and tried to keep them away from the demonstrators.
The authorities had jammed the transmissions of American-based TV and radio stations run by Iranian anti-regime exiles, who had in the past directed people to protest sites.
The authorities also apparently disabled the mobile phone system in the immediate area of the demonstration, and closed down the university's campus earlier.
In the disturbances last month, vigilantes attacked protesters with clubs, chains and knives, causing many injuries and provoking bitter protests from student leaders and some reformist politicians.
Neither the student groups nor the reformist movement are supported theses protests, despite their own great unhappiness with the current situation in which a hardline minority has been blocking the changes for which many millions of Iranians voted, our correspondent says.
Many student leaders were also detained in advance of the anniversary.
Three of them were marched off with pistols to their heads by bearded gunmen after giving a news conference on Wednesday morning.