You can not just go somewhere without a permission and occupy other peoples property.

You absolutely can and people routinely do.

And then you are regularly arrested for trespassing and damage to property.

On the other hand, you can simply register the demonstration with the municipal office and then demonstrate in the designated area as often and as long as you want.

Shame on you if you tell people to disregard the law, ending them up in jail.

Chose your poison.