Their behaviour is totally inhumane and unacceptable. First of all, in an office where everyone has day to day roles to fulfil and tasks to complete, how someone can find time to observe a person, who is actually doing a job. If I were the boss, I would question the observer, how did he find time to observe you and then see the demon... It means that these people making comments and observations have plenty of free time... That's one aspect of the situation.
As for their rude comments, I think it will be very difficult to prove that they really said that and they will disown their words straight away. If you want to proceed against them, in my opinion, you should try to get videos of any surveillance cameras (if these captured your conversations with them) or else invest in those spy camera pens to capture something as evidence. This will help you build a proper case against them and it will be a valuable lesson to them and the likes of them.