If you strolled down the Southbank in London on Friday, you may have noticed pieces of brightly coloured knitting tied to various objects, or perhaps your bike lock was covered with wool. Welcome to guerrilla knitting.

This is graffiti, but with yarn. It's nothing new - people have been doing it for years, all over the world - but Friday's session was special. It was the first time that Magda Sayeg, one of the craft world's most revered guerrilla knitters, 'tagged' London. The 35-year-old from Texas is the founder of Knitta Please, one of the first guerrilla knitting crews. Sayeg started out decorating her local area with leftover pieces of knitting from unfinished projects. She soon got more ambitious, travelling around the world and leaving bits of thread behind: she counts the Great Wall of China, a handful of Paris landmarks and the Welcome to Manhattan sign on the Brooklyn Bridge among her greatest conquests.

What's the point, though? "It's about making people smile and bringing art out of the galleries so everyone can appreciate it", she says. "I love it when a postman, who has driven past the same stop sign every day, suddenly sees it tagged with knitting and emails me to say how awesome it is."

Magda Sayeg in action

But back to Friday's antics. Magda was in town to talk about her latest project, a crochet-covered bus in Mexico, on the Richard and Judy show. She fancied some impromptu guerrilla knitting, also known as 'yarn bombing', and I was happy to oblige - as were a handful of ladies, aged from 24 to 64, from the Stitch and Bitch London knitting group.

Logistically it wasn't like Magda's typical outings, where potential targets are identified and measured first, and cosies are knitted to fit later at home. Instead, we brought bits of knitting we had going spare and clicked away furiously for an hour before tagging commenced. The railing by the skate park was the first victim, much to the amusement of the teenage onlookers. Statues, lampposts and bollards were next, with each swatch either stitched in place or secured with zip ties. Tourists took photos and asked questions, and when we left I spotted one woman scouring the plaque underneath a statue, trying to work out why it was sporting a pink leg warmer. How long the knitting will stay on the Southbank is unknown. Magda reckons that some of her work is still intact after years, while others have been taken down by people who won't tolerate even fluffy vandalism. One piece even turned up on eBay. "They sold it for $14", she explains. "I don't mind, but it was definitely worth more than that."

Useful links

Find out more about guerilla knitting and crochet on these great websites, blogs and picture-sharing groups.

Knitta Please

Stitch and Bitch London

Knitted landscape

Knitwit

Knit sea

Knittups

Outdoor knit

Stikkontakt

Flickr

Yarnbombing