With Khamenei's speech yesterday the political stage is set and now the real clashes have begun:
Eyewitnesses described fierce clashes near Revolution Square in central Tehran after some 3,000 protesters chanted "Death to the dictator!" and "Death to dictatorship!" Police responded with tear gas and water cannons, the witnesses said.
Simple rioting will not achieve anything but a sharper crackdown.
To morally win, the demonstrators will have to convene peacefully in one huge crowd in one place. The police will try to prevent that by blocking the access roads to the possible places in question and by disbanding any infant formation of a bigger crowd.
Having been in several bigger conflicts with riot police forces I see no chance for the demonstrators to achieve that goal. In a determined demonstration one can defend the ground one has if one is willing to take and inflict some casualties. But even with violence it is nearly impossible to get to an area like a big prominent place if a competent police force is aware of that goal and is willing to employ the usual anti-riot measures like teargas and water cannons.
The policeman in this picture seems confident and competent to me.

I'd think twice about outfighting this guy.