The first thing to do is take a few deep breaths in case you have, or are being, assaulted on the basis of your race. The purpose of such a comment is to upset you. Don’t let that happen. Stay calm and don’t let a petty racist dictate your state of mind.
Once you have gathered your cool, you can either choose to ignore the assault or respond to it in a befitting manner. But be aware that choosing the latter may end up in a serious brawl because the racist will want to respond back.
If the racist remark is carried out with a violent physical act, it is best to defend yourself and immediately seek help. Don’t let the nutcase strip you off your dignity. Show grace at all times and report the matter to the police at your earliest. Try and get away from the situation, and if you can’t, fight back as best as you can and stay firm. To know about the nearest Metropolitan police station follow this link.
In case the victim is somebody else, depending on the situation, you should try and help them. If the racist and victim are in a fight or in imminent danger, immediately call 999 and inform them (the emergency number of Met Office). But if it was a comment and a one off incident, suggest the victim to ignore it.