You shouldn't have problems. This issue is object of local laws. The local law states, more or less, that if you get money from "occupying" public places then you should pay a fee. If not (this is your case) you cannot be asked for anything.
Here is a forum about this (In Italian)
Anyway you could be asked also to move away (Yes, Italy is a strange place) although this is very rare.
If this is the case. Close your tripod, get a coffe/gelato/pizza/carbonara/ go back to where you were. The local policeman will have gone.
Place your tripod and take pictures. The chance you have problems for this is very very low.
In case you need analog stuff visit this site:
Have fun