The call it Street Wars, and the rules are simple: Players are given the name, picture and target of another player, and the goal is to squirt them with a water gun. The game has been played in New York City, Vancouver, Vienna, San Francisco and Los Angeles. But when London residents organized a game, they got a stern warning from police. "Calls to the police may lead to disruption to services and the deployment of armed officers. Any offences committed will be treated seriously," the British Transport Police.