Disturbances are reported to be taking place in Tehran, as protesters marked the anniversary of a raid on a student dormitory.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.
The raid in 1999 by police and right-wing vigilantes triggered several days of violent street riots.
As darkness fell on Wednesday, large numbers of people began congregating in streets and parks close to Tehran University in the centre of the city.
The campus itself had earlier been closed down - one of many measures taken by the authorities to try to head off possible trouble on the anniversary of the riots.
Tear gas
Hundreds of security forces and riot police also flooded the area.
There were reports that clashes later broke out, with tear gas being fired to disperse protesters.
One report said right-wing vigilantes were also present, but the official security forces clashed with them and tried to keep them away from the demonstrators.
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 organisations nor the reformist movement are supporting these street protests, which have been dismissed by regime hardliners as hooliganism and American-inspired agitation.
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.
But many other people have still taken to the streets.
Some of them may be hard-core opponents of the regime, others frustrated reformists.
But on such occasions, large numbers of those who turn out are simply curious to know what is going on.