Hopefully it well never happen to you but it might. Although it's a lot less likely than some people would like you to believe, your erotic power exchange activity may for some reason attract the attention of other people who, in their turn, will attract the attention of the police. We cannot stress enough that this hardly ever happens, but when it does, you can save yourself a lot of trouble by following a few simple guidelines: first and foremost by understanding that it is NOT the officer's fault he or she is there and secondly that he or she is not necessarily the enemy.
A lot of the misunderstanding between law enforcement and the BDSM community as well as individual BDSM people is the result of very poor information, strong stories and urban legends. Plus there may be a social stigma on erotic power exchange, a large part of the erotic power exchange community in its turn does a pretty good job projecting stigma and prejudice on others, most significantly on law enforcement.
Let's make a quick end to all sorts of legal speculation and feed you with some straight facts.
Sexual-related activities with anyone underaged, including erotic power exchange, are always illegal. The definition of underaged depends on where you are, but 18 or 21 usually will be the age of consent. You may not like that, but that is the law and if you want to change that, join your local political party but DON'T blame the police.
Whatever professional dominatrixes may tell you (in a debatable attempt to protect themselves), erotic power exchange IS sexual activity in the legal sense of the word. Hence, if prostitution is illegal in your area, visiting a professional dominatrix may be illegal and you may open yourself up to legal risks. Again, don't blame the officer, he or she has only one job: upholding the law - not making or changing it.
In all South American countries erotic power exchange is illegal or at least not socially acceptable. In most Middle East countries it is as well and it is anyway if you are NOT a Muslim and play with a Muslim partner prior to marriage. In Asia the situation very much depends on the dominant local religion. If that is Muslim, assume problems. In continental Europe erotic power exchange between consenting adults is not illegal, however doing it out in the open may be considered an offense. The UK is NOT continental Europe and in the UK erotic power exchange is illegal, although it may not always be prosecuted. As is the situation in Canada, Australia, New Zealand and all other Commonwealth member states. In the USA the situation very much depends on what state you are in, but it is likely to be legal, or at least not specifically illegal, although sometimes not really socially acceptable.
Play parties in almost all areas may sometimes be illegal for non-BDSM related reasons, usually because of the violation of liquor related local legislation. Not all party organizers educate themselves well enough. Make sure you do and ask.
The most important thing to do is to stay calm. If you are the dominant, take care of your partner first. Offer the officer(s) a chair, accept that they will want to make sure you are not going to run away and ask them to wait a few minutes so you can take care of your partner and - if you need to - either put on some clothes or change into something more appropriate (in other words, get out of your kinky outfit if you are wearing one).
If you are wearing a kinky outfit, think about the image you are projecting. The officer(s) are trained to look for certain signs of potential violence. Like it or not, a leather jacket, spiked collars, whips on your belt, chains and handcuffs are such signs. Whatever you try to explain, more than half of what you are about to say will make very little sense due to your studs, spikes and boots. In all fairness, to the officer you look like Kermit the Frog explaining the United States foreign policy. Changing clothes and more importantly re-assuring (and untying) your submissive partner is important and will buy you time to calm down yourself.
Next, people who are into erotic power exchange should educate themselves and be aware of the dilemmas (sketched elsewhere in this section) that law enforcement officers are confronted with. There will almost always be a need for education, but one can hardly expect the officer in question to rely on information from what he/she sees as an offender or victim. Hence in a crisis situation such as an arrest, do not try to educate, because it will not help. Instead, try to understand a few simple facts:
officer involved has every right to see that as trying to talk yourself out of the situation. They cannot help that. For very good reasons they are trained that way and when you yourself are the victim of a fraud scheme or a rapist, you will be glad they are trained the way they are;
law enforcement officers are almost always in a hurry. The main reason for bringing you (or asking you to come) to a station is because that will allow others to take over AND create sufficient time for explanation. Their job is to be on the streets; others do the talking. Plus, the officer involved has been trained not to take risks. Talking to you may very well be a legal risk for the officer that, at a later stage, may backfire on him or her personally. Again, officers have been trained not to do that and procedures do not allow them to;
the officer will be wise to explain his/her situation briefly. That leads to mutual understanding of the situation but do not assume he or she will or even can. If he/she does not do that, ask.
If you, as a couple or individual, are confronted with a situation like this, you may want to point the officer(s) in question to external neutral sources for information (such as this one). In any case: always point your lawyer and police officers to organizations that have experience with assisting and educating lawyers and law enforcement officers in erotic power exchange related cases. The Erotic Power Exchange Information Center has a long track record in many different countries, but there other resources as well. In Canada for example there is the Toronto Safer S/M Project. For the USA, the National Leather Association, a nationwide organization with a separate law-project, may also be a very good choice.
Finally if all this happens in a club, bar or during a munch or event and you are the person responsible: this is NO time for activism. The officers involved will not go away because you tell them to and they have neither the time nor the interest to listen to your arguments. Be reasonable, explain and do the talking at the appropriate time and place to the appropriate people. In the meantime, attend to your guests and use your position to make sure the situation is being dealt with professionally and quietly.