Iran's Supreme Leader Ayatollah Khamenei has warned student protesters not to expect any pity from the authorities in the country.
Ayatollah Khamenei blames the US for the protests
He also accused the United States of stirring up trouble by trying to create a division between the regime and the populace.
The numbers at the protest swelled after broadcasts by US-based satellite TV channels for people to gather at Tehran university.
But in an address to a crowd of several thousand in the town of Varamin in southern Iran, Ayatollah Khamenei said America was trying to turn disgruntled people into mercenaries in order to cause trouble.
He appealed for calm, but also said if necessary he would call on his supporters to deal with the troublemakers.
The ayatollah said: "Leaders do not have the right to have any pity whatsoever for the mercenaries of the enemy."
His speech was broadcast hours after the end of a demonstration in Teheran which turned violent after vigilante groups clashed with protesters.
According to reports, clashes erupted between rival groups, with both sides throwing stones at each other.
Militant groups also arrived on motorbikes, wielding chains and batons, and hundreds of riot police were called in to try to disperse the demonstrators into the surrounding streets. According to reports, at least ten people were arrested.
About 3,000 demonstrators had gathered outside the university campus to support a student protest against plans to privatise Iran's colleges.
They shouted slogans against the clerical authorities, including "Death to Khamenei!", and for President Khatami to resign.
Others shouted slogans such as: "Tanks, artillery and guns no longer have any power."
'Fed up' students
Many people have lost hope in Mr Khatami, reports the BBC's correspondent in Tehran, Miranda Eeles.
After voting him in six years ago on a platform of reform, Iranians - especially the young - are fed up at the slow pace of change.
The authorities are very anxious for the situation not to get out of hand.
They have stressed to students not to repeat the events of 1999, when clashes with law-enforcement officers lasted for three days and left at least one student dead.
That crackdown marked the worst anti-government clashes since the Iranian revolution in 1979.