There are two types of offences, cognizable and non-cognizable. The cognizable ones are the serious crimes, say, crimes against persons (murder, rape, kidnapping, extortion etc). Non-cognizable ones are the not so serious ones - cheating, forgery etc.The police can arrest without warrant in case of a cognizable offence only. However, the police does not have the right to beat you or hurt you after arresting you no matter what offence you are booked for. That is not what the law prescribes.