If it's a good cause, you don't need a permit. If they arrest you, you must have been causing a disturbance or trying to hurt someone.
Otherwise, if you are peaceful and well behaved, you are allowed to say what you want where you want. If things get out of hand, you should leave. No one wants to be in an angry, unruly mob, even if they originally gathered together for a good reason.
It's best not to wear masks, or carry weapons or drugs. Unless you are protesting your right to wear masks, carry weapons or drugs. They will probably not give you a permit for that even if you ask.