London - On a bright Sunday afternoon in central London, in a low-lit converted wing of a shopping-mall cinema, hundreds of fans cheer as two teams of 20-somethings riddle each other with bullets during a big-screen game for a $25 000 reward.The game is Call of Duty, a hugely popular first-person shooter that demands hair-trigger "twitch" reflexes, teamwork and hours of daily training to beat the best in the fast-growing world of competitive gaming tournaments.So-called "eSports" are set to generate almost $500m a year in revenues globally by 2017, according to research firm Newzoo.The weekend tournament is just one of two dozen events this year at London's "Gfinity Arena", billed as the only dedicated eSports venue in the country, seating 600 people across two screening rooms. That may not seem much next to a 40 000-capacity football stadium, but millions also tune in online."Call of Duty is a very important game for us," said Neville Upton, head of the British eSports firm Gfinity that launched the venue this year. Citing 2.5 million views for previous tournaments, he added: "[Most] people who watch are young males... sponsors are keen to hit that demographic."The atmosphere veers from the quiet tension of a chess match to the quick-fire highs and lows of a Formula One race.Commentators make sense of the on-screen chaos; the blink-and-you'll-miss-it kills average at one every 1.5 seconds.The players, many of whom have attained celebrity status, sit hunched over screens in transparent booths and barely register any emotion as the on-screen body count racks up.The stars of the evening are undoubtedly Optic Gaming, probably the best-known eSports team, who wear distinctive green-and-white team shirts and whose fans wait patiently by the cordoned-off players' lounge for an autograph and a selfie."This is our first event, and it's pretty good... Better than when you see events streaming [online]," said audience member Ian Curley, 18, waiting in the wings to catch a glimpse of 19-year-old Optic captain "Scump" - real name Seth Abner.After a ritual post-victory cigarette and snaps with fans, the red-haired Abner and his 20-year-old teammate Matthew Piper head for the lounge to recline on bean-bags.Being an eSports athlete takes hard work, said Piper. Training takes 6 to 12 hours a day and careers are brutally short given the need for razor-sharp motor reflexes and hand-eye coordination. Retirement age can be 23."The biggest thing is team chemistry... It's important to be able to hang out every day," said Piper.The prize money of $25 000 is small change compared with the $1m earned at world tournaments; Piper admits his team is really there to win "bragging rights" after a shock defeat at the world championships earlier this year.For Gfinity's London venue, though, the event is crucial to making eSports a household name in Britain and Europe."I am looking at the possibility of different cinemas, different sizes... potentially bigger venues for some games," Upton said. "The market is growing rapidly."