I once organized a protest against a pipeline in my country, what I did was start a facebook event that people can rsvp to so that you know what size the crowd is roughly going to be. Share, share, share, and share some more. Stress on the page that is will be a peaceful and civil protest meant to draw awareness. Contact the local media and let them know you are planning on doing this, that way you may get some coverage or interviews to help spread your word even further. If the numbers for the protest seem large I would contact the local police department and just let them know that you are planning this protest and it’s looking big. That way they can have people nearby on call. I hope this was helpful. Good luck :)