Police are warning of criminals posing as undercover cops and "fining" tourists for bogus minor offences ahead of the London 2012 Olympics.
They say gangs from South America and Eastern Europe are targeting visitors, accusing them of littering or not having ID documents.
They then demand an immediate cash "fine" or accompany them to cashpoint machines and insist they withdraw money.
Scotland Yard detectives have arrested four suspects, but believe there are many others trying to dupe confused tourists.
Detective Superintendent Nick Downing said: "It's theft, pure and simple. They prey on foreigners who don't know how our police operate and have intimidated some to hand over money.
"Visitors should know that no police officer will ever ask for cash in those circumstances."
Police have also warned sports fans to beware of eight bogus Olympic ticket websites that are either unauthorised or fraudulent.