High noon in London as water pistol sharpshooters take to the streets

They may be glancing around with a wariness more commonly associated with someone who has double-crossed a Russian mafia boss or a Colombian drug lord.

But for 150 Londoners who have paid £40 to take part in StreetWars - the squirt-or-be-squirted urban fantasy game in which rival players attempt to "take out" targets with a souped-up water pistol - the tension is all part of the fun.

The game, which commenced at midnight yesterday and will run for three weeks or until there is a single survivor, began life in New York and Canada before migrating westwards to California. Now it has arrived in the UK.

It is the creation of Franz Aliquo, a 30-year-old equities lawyer from New York, who goes under the name of the Mustache Commander.

He sees it as a chance for grown-ups to live out their fantasies. "It is three weeks living in an action movie rather than watching one - a perfect escape from the trials and tribulations of real life," he says.

But of course not everyone sees it that way. In Mr Aliquo's home city, the Mayor, Michael Bloomberg, has called on participants to "grow up". In London, players have faced accusations that they pose a public menace, playing their game just a year after police shot and killed Jean Charles de Menezes at Stockwell Tube station.

Police warned players not to carry water pistols that could be mistaken for real weapons, while the Mayor, Ken Livingstone, also expressed concerns.

But for Billy S - not his real name - a thirtysomething City IT specialist, talk of danger was overblown.

He spent the morning from 4am lying in wait for his target outside his London flat. Dressed in a business suit in order to blend in with his surroundings, he shot but narrowly missed but then as his quarry fled, he trained his fire on another member of the rival team who popped her head out of the window to see what was occurring. She too survived.

He points out that under the rules, public transport is out of bounds as are bars and restaurants and places of work. Water pistols must also be brightly coloured to stop any confusion. "The rules are very clear. Nobody is out to seriously scare anybody or do anything stupid," he said.

While the winner will pocket £500, the event is all about having fun and the thrill of the chase.

Organisers insist that StreetWars is not similar to stalking. Players must be over 18 and "not live with their mamas". But it is still a game of hardball.

In the United States, one player lured an opponent to a fake job interview before literally "hosing" them down. Others have rummaged through rubbish bins for clues about their targets or tricked their way into apartment blocks.

Previous games have all passed off without serious incident and organisers now want the game to become global and permanent.

The individual "missions" were revealed to the players under the cover of darkness at a City hotel on Sunday night. The target's name, mobile and address was handed to their would-be assassin. The rest is now up to them.

Hamish Moseley, 28, film executive: 'There are a lot of people out there looking for me'

Hamish Moseley had taken the precaution of wearing a wig on the way to work yesterday. Having been summarily appointed team leader of his "assassination squad", Death Aquatic, he was a high-value target for would be assailants. If he is "killed" the entire three-man team is out.

"Frankly I'm crapping myself. I was looking outside my house this morning to see if anyone was there. I wore the wig when I went to the pub at lunchtime and now I'm just keeping a low profile. The attraction is living on the edge. The thrill is going up to a complete stranger you only know from a photo and soaking them. But you do think, 'what if I get someone's surly brother by mistake and they beat me up'?"

But the tension was already starting to show. "It is only one day into it and I'm having a bit of a miserable time at the moment hiding here in the office. I'm worried that there are a lot of serious people without jobs out there and a lot of time on their hands who are looking for me."