By Sadeq Saba
BBC regional analyst
As anti-government demonstrations escalate in Iran, the hardline authorities are using Islamic vigilantes to suppress the students and their supporters.
Students have been attacked with knives
During the fourth night of protests in the capital Tehran, armed plain-clothes groups raided university hostels injuring a number of students.
They attacked other protesters with clubs and chains.
The militants pledge loyalty to Iran's supreme leader, Ayatollah Ali Khamenei.
Conservatives in the Iranian leadership for many years have been using hardline Islamic groups as one of the most effective tools to suppress any challenge to their rule.
Whenever there is a pro-democracy gathering, members of this shadowy organisation are brought to the scene.
They normally arrive on motorcycles, wielding knives, chains and clubs.
They are young men with distinctive features such as beards and untucked shirts.
Their sheer presence causes panic. They attack their opponents viciously.
It is widely believed in Iran that the vigilantes are paid and organised by people close to Ayatollah Khamenei.
They are fiercely loyal to him, and see their role as defending Islamic values and the Iranian revolution.
Because of their powerful connections, they feel free to openly attack their opponents without any fear of prosecution.
They are not like the normal police force, which is accountable for its actions.
And the hardline authorities often portray these militia forces as the true representatives of the Iranian people.
But most Iranians regard them as young thugs.
Therefore, as general discontent increases in Iran the action of these militant groups could backfire.