Where are the tanks?

While this has certainly been horrible the carnage hasn't reached the level of Tiananmen yet and there must be a reason for it.  China '89 didn't produce these kinds of images of riot police throwing stones and running away in fear from mobs of people.

The mullahs have more than enough firepower at their disposal to crush this in a day and clearly they have no qualms about murdering people in the street so it must be something else.  Could it mean that the military will not participate?  If the best they can do is send helmeted police into the street to swing sticks at thousands of people willing to die then it is already over and the only remaining issue is how soon will they be out of power.