How to deal with the fake police scam in Spain?

Follow the steps:

1. Remain calm. Ask for identification before going anywhere with them or giving them anything.
2. Don't sign anything without a lawyer present. If they start accusing you of anything, state that you require they then contact your embassy to help you with a lawyer. Generally if they're scamming, they don't want documentation or third parties involved.
3. If they want to search your bags, ask if you're under arrest or have done anything wrong. If you're not under arrest (as you'd hope) then ask why you can't just walk away right now. If they want your ID, agree to do so at the station with a lawyer or embassy member present.

Basically, your goal is to throw documentation, process and evidence at them - all of them being something scammers will want to avoid.

Much of this is learned from dealing with (real) police who were corrupt and doing basically the same thing. In theory, fake police should be easier to deal with. Good luck, and hope it doesn't happen to you! Spain is a great, friendly country in general.

I have been in similar situations with people trying to pretend that they have a legitimate reason to pressure you to do things that might be taken advantage of.

What I do is that I immediately pick up my phone and start dialling the police. When they ask me what I am doing I am honest and tell them that I am phoning the police, and say that they might be well within their right, but I am contacting the police to get instruction on how to proceed.

So far my only response to this is that they try to defuse the situation, or simply run away. So far I haven't been able to dial the complete number.

In any case, it is a very good idea to get to know the phone numbers to the local authorities. That might help you in many more difficult situations.

Oh, I found a good suggestion in the Barcelona tourist guide

"I'd like to warn your users about an all too common scam which is unfortunately being used on the streets of Barcelona. This happened to me on 18 May, and I think you'll be doing your users a great favour by alerting them to this attempt to steal goods and possessions. It works like this: A young man comes up to you and asks for directions to a popular landmark - e.g. the Gaudí Museum or the Sagrada Familia Cathedral. As you're explaining or just saying you don't know, 2 or 3 men come up to you both and say they are police, and flash an ID card at you that looks like a police ID. They ask if you know the person that asked you for directions. They then explain that they have been trailing this man under suspicion of drug smuggling and that you are now also a material witness. They ask for your ID. When you take your wallet out of your pocket or bag they assault you and run off with the bag, passport, or whatever they can get their hands on. My advice is don't ever stop and discuss anything with anyone you don't know - if this happens to you, you just say 'sorry I don't know' and walk off very quickly and don't stop - like I did! "