If they are entering a restricted area inside of City hall. You must make it clear to them that the area is being restricted before detainment.
They have an appointment to speak to the Mayor.
If you have been forced to detain them for their conduct towards the Mayor.
They are in a Public space around the Mayor and have not shown any signs of aggression towards them or behaved under circumstances which do not directly incriminate them.
If you suspect them of possession of Narcotics or weapons under circumstances which do not paint them as a threat to the Mayor's safety. Please notify Police of suspicious individuals so they may investigate.
Because you find them "suspicious" without reasonable grounds to do so.