Last year in June when President Ahmadinejad was elected to serve a second four year term, hundreds of Iranians wearing green filled the streets to demonstrate and show their disgust. Then suddenly they were nowhere to be seen.
The Basij is a paramilitary volunteer militia founded by the Ayatollah Khomeini in November 1979. They are supposed to be subordinate to, and receive their orders from, the Iranian Revolutionary Guards but have been described as "a loosely allied group of organizations including many groups controlled by local clerics".
Iranian youths volunteer to join the Basij, often in exchange for official benefits and are loyal to the supreme leader Khamenei. They currently serve as an auxiliary force engaged in activities such as internal security as well as law enforcement, organizing public religious ceremonies, and more famously - morals policing and the suppression of dissident gatherings. Basij are present in schools, universities, state and private institutions, factories and even among tribes.
So you could say the Basij are the average citizen's worst nightmare, looking over every shoulder. The question now is whether the Revolutionary Guards can control these young religious zealots and convince them that killing their brothers is wrong.