I’m not a lawyer, but I believe in most cases you’re free to demonstrate without a permit, see here: www.aclu.org/know-your-rights/protesters-rights I believe if, for example, you exceed the area’s capacity the police are supposed to give an order to disperse and then give you time to disperse.

However it’s better to get a permit, agreed. And the police may not do what they’re “supposed” to do.