Protest of one interesting tactic to organize a powerful protest without a permit (by https://extinctionrebellion.uk/)

You have to place a person with a sign every 50 metres from each others so that when people walk from point A to point B (let us say a central square and a pedestrian shopping street, places which are frequented by many people) would keep seeing a protester sharing their concern.

It is also a good way to keep the police from seeing it as a concentration of people. So you can do it without permit.