Ian Black in The Guardian:
Iran's basij militiamen do not wear uniforms or insignia, but they are still easy to spot on the streets of Tehran and other cities. With their short hair and camouflage jackets or trousers, and armed with batons, knives, iron bars and chains, they are the shock troops of the Islamic regime as it struggles to contain the biggest wave of unrest since the 1979 revolution. Basiji have been “in action” for the last week, beating protesters without embarrassment and with impunity in broad daylight.
Basij (the name means “mobilisation”) are commanded by a senior cleric but are subordinate to the Revolutionary Guard Corps, which in turn answers to the supreme leader of the revolution, Ayatollah Ali Khamenei. The fatal shooting in Tehran's Azadi Square during Monday's massive protest march — the peak of the unrest so far — arose from a clash between basiji and pro-Mousavi demonstrators. Basiji are also said to have attacked students in Tehran University dormitories, along with police. Seven other people were killed, apparently also with the involvement of the militiamen.
Basiji are mostly young men from poor, religious families, but there are older volunteers too. Membership brings privileges in the form of guaranteed university places and access to certain jobs.