When can demonstrators be arrested?

Answer 1

As with most things concerning the law - it depends.

What is the definition of "a demonstraion?" What is the size of the "demonstration" (2 people? - 2,000 people?). Where is it being held (e.g.: grounds of Embassies, Consulates, hospitals, etc. are clearly off-limits).

'Ad hoc' demonstrations can be controlled by the decisions of lower level supervisory ranks, but as a general rule the larger a "demonstration" and the more it disrupts the functional operation of the public at large, such decisions are usually made by the political office-holders of the municipality. When they determine that the situation is, or has the potential of, becoming out of control the police are ordered to disperse it.

Answer 2

Never