LONDON (Reuters Life!) - Their surreptitious strikes can happen at any time of the day or night.
They target the urban landscape -- fences, poles, bicycle racks, construction sites and other commonplace fixtures of city life -- but their weapon is wool and their goal is to enhance cities with their creations rather than destroy.
Over the past few years, a London faction have adorned various famous structures in the British capital with wool, including scarves for the lions of Trafalgar Square and a cover on a red telephone box in Parliament Square.
But they can strike anywhere and on Saturday, June 11th this global network of "graffiti knitters" will target objects in local areas that they want to enhance with their handiwork, as part of the first International Yarnbombing Day.
Originator Joann Matvichuk, 44, of Lethbridge in the Canadian province of Alberta, came up with the idea for a special day as she tossed and turned in her bed one night unable to sleep due to insomnia.
Matvichuk, who says her motivation for graffiti knitting is to make people smile, set up a Facebook fan page in the middle of that restless night, and the group now has more than 2,000 members from all over the world, including India, Germany, Scotland, Australia, New Zealand, Spain, Italy, Portugal, Egypt, Iceland, Israel, Britain, Canada and the United States.
"I live in Canada where we always jokingly say we have winter 10 months out of the year," Matvichuk told Reuters.
She said making something that's beautiful and colorful and then putting it out for people to see is a great way to brighten up a gloomy day.
"You'd be amazed at how a little crocheted daisy left on a chain-link fence can make people smile when they see it."
Leanne Prain, 34, from Vancouver, British Columbia, is credited with coining the term "yarnbombing" in a 2009 book of the same name she co-authored with Mandy Moore.
"We consider it positive graffiti, it's not there to be ill-intentioned, or to give anyone a bad day," Prain said. "It is supposed to be a force of positivity in the world."
These guerrilla knitters eschew stereotypes of their craft as a past-time for the old or anti-social and frustrate efforts by some critics to burden their art with political overtones or diminish it with comparisons to the kind of urban graffiti made by artists such as Britain's Banksy.
Lauren O'Farrell, who operates under the alias "Deadly Knitshade" in the "Stitch London" group said the main goal remained just to have fun.
"A lot of people are desperate for us to have these political meanings underneath, and all these angry opinions, but we are showing off our craft."