Now that we have covered what not to do, we can cover what you need to do, and what your end goal will be.
The very first thing you need to do is get this on the record. Having a witness present helps, but a recording of the incident is very powerful evidence. Audio with visual is the best evidence you can collect and will be used as your affidavit when you begin your counter claim.
CCTV and smart phones are great tools for this. Turn them on before any encounter.
Language - What to do