If you are witnessing a dog being beaten, have you called the RSPCA? The dog is probably scared stiff, hence the reactivity x
As Hannah has suggested I would report to RSPCA if concerned about how the dog is being treated or the police if you believe it poses a threat to the public. Dog attacks by powerful dogs such as pitbulls have devastating results and if the dog is stressed it's far more likely to show aggression.
I’ve reported it to the police as a dangerous dog. RSPCA told me they won’t act unless the dog is malnourished I’ve had this problem before x
Thank you x