You're right, it would require martial law to express this kind of authority. However most people won't be opposing this and the main battle is against uncertai

You're right, it would require martial law to express this kind of authority. However most people won't be opposing this and the main battle is against uncertainty and lack of information. Government power is relatively fragile anyway, and in times like these, this kind of notice can settle fears and give people some direction. They'll follow it given the alternative of chaos, and that's all the city needs.