The e-mail had said to look for a man with a moustache outside Liverpool Street station in London, but when David Styles arrived a little after 7pm three Sundays ago he was surprised to find a 6ft Charlie Chan lookalike.

“I understand you have got a package for me,” said Styles.

“Go to pub. Someone contact you,” said Charlie Chan.

Outside the pub opposite, Styles was approached by an American gangster in a suit with wide lapels. “Follow me,” he said.

He led Styles through the bar to a back alley, round the corner, up some stairs, in and out of buildings, until Styles was finally ushered into a dim hotel room.

In the shadows were assorted goons and molls and on the sofa, wearing powder blue, sat Mr Big proffering a large bottle of Jack Daniel’s.

“This is assassin juice,” he said. After they downed a couple of shots Mr Big handed him a beige dossier containing details (photograph, telephone number, home and work addresses) for the person he had to “smoke” and a recommendation for where to buy his weapon: Toys ‘R’ Us.

That’s right — Toys ‘R’ Us, where the following day Styles purchased a foot-long DayGlo Super Soaker, all of which makes you wonder: is it merely kooky or downright socially unacceptable for a 36-year-old IT consultant from south London to run around town with a water pistol pretending to be a thug? Apparently it is neither. Styles has simply joined the growing number of adults who get their jollies playing high concept games on the city streets. Many of these games are facilitated by an evolving internet and new generation mobile phones that allow players to locate one another in public.

Flash-mobbing (whereby people texted each other invites to impromptu urban pillow fights and dancing sessions) gave birth to the trend a couple of years ago and now interactive, partially digital frolics that unravel in the real world abound. Gangsta War, for example, requires players to scamper about picking up virtual hot goods; or there is BotFighters, where you create your own online robot and then fight other robots with your mobile phone when they come within a mile of you. Global Positioning Systems, like satnav in cars, make this possible.

The result is a population gripped by the kind of whimsy not seen since Kit Williams and Masquerade in 1979. Styles’s preferred brand is StreetWars: Killer, imported to London from America this summer. A couple of hundred participants were given the details of targets and then, crouched in office car parks or the toilets at All Bar One, they leapt out to soak their quivering prey, thus eliminating the targets from the game.

When news of StreetWars: Killer reached Superintendent Bob Pacey of the British Transport police, he opined that running onto the Tube brandishing a mock gun could cause “passengers and staff genuine fear”. But this has not held back Styles and co.

His first victim three weeks ago was Joy Lo Dico, a journalist. “I got to her house in Hammersmith at 5.30am and hid in a bush waiting,” he says. “Then at 8.03 her StreetWars team-mate left for work so I chased him down the street. He got so scared that he dropped his work bag and just kept running.

“Eventually my legs gave up and I stood there panting. People were saying, ‘Are you okay? Did he rob you?’ It took four days in the bushes opposite Joy’s house to finally get her,” he grins.