In any country, police personnel are supposed to be on your side. However, it's also easy for scam artists to pose as police -- and use that implied authority to take your money. Here's how they do it.
Most swindles take place in any popular tourist destination. If you're a tourist, you're probably not familiar with the typical law enforcement uniforms, so it's easy for anyone to put on something that looks official. In this scam, a person claiming to be a police officer approaches you. He will tell you there's been a problem in the area with counterfeit bills and ask to check your wallet for any offending currency -- all under the pretense of "protecting" you.
The fake police officer will then rifle through your wallet, giving it back to you with an "all clear." He'll be long gone by the time you realize some of your money is missing. Variations on this scam include fake authorities at train stations, roadblocks and other official sites. Sometimes they'll just grab your wallet or passport and run.
How do you avoid falling victim? Don't voluntarily hand your wallet or passport over to anyone, ever. Know what the area's law enforcement uniform looks like. Ask for identification numbers and names if in doubt about someone's identity.