In Shoreditch, far from London's beaten tourist track, a group of visitors is staring keenly at the graffiti-covered gates to an abandoned construction site.
Their guide, Karim Samuels, points out the black-and-white images of two young faces, and behind them, a piece of street art by Britain's most famous and enigmatic street artist, Banksy.
Sadly, the Banksy of a rat wielding a knife and fork is too precious to stay public and has been hidden under boards to protect it from vandals and the weather, Samuels tells members of his Street Art London tour.
The owner plans to make a feature of the image when he opens a new restaurant at the site, capitalising on a surge in popularity for Banksy's work.
"If you're lucky enough to have a Banksy piece on the corner of your house, it will probably boost the value,'' Samuels said, his beanie hat pulled down low against the freezing wind.
Street Art London leads visitors through the maze of Shoreditch's colourful streets, showing off the area's most outstanding works.
The tours transform east London into a vast outdoor museum, where many of the works on show contain hidden messages - and quite a few of the artworks are illegal.
"Art is not inside the museums or the galleries. Art is everywhere you can imagine,'' said 20-year-old Brazilian student Felipe Rama, donning 3D glasses to enjoy a psychedelic portrait daubed by Australian artist Jimmy Cochran on a cafe wall.
Urban art needn't be limited to spray-painted walls and shop shutters. It can take the form of mosaics, sculptures - or even painted blobs of discarded chewing gum, which might be trampled over without the help of a guide.
"Since 1998, Ben Wilson has painted between eight and 10,000 of these,'' said Samuels, bending over one of the tiny bits of street art stuck to the pavement.
"He got arrested, but the authorities came to the conclusion they could not charge him with criminal damage - because he's not actually damaging anyone's property.
"It's a bit of chewing gum, it's litter on the street,'' he added, brushing away the snow to reveal a tiny piece of gum painted to look like a nearby crossroads.
He points out some miniature dancing figures at the foot of a nearby wall, painted by Mexican artist Pablo Delgado, and a giant bird on the side of a building.
"The artists see the city as a kind of playground,'' Samuels said, adding: "Where there's a wall, there's a way.''
It's a philosophy claimed by many street artists looking to conquer new territory.
Some graffiti is wiped clean by the government, but other works are destroyed by artists who are simply jealous or want to re-use a space.
Some also disappear during construction work and some rarer pieces are commissioned by a building's owners.
"The street is a jungle,'' said Samuels. "It's like this huge war going on all the time between street artists, graffiti writers, the council, the police, advertisers.''
As a result of the neighbourhood's ever-shifting decor, the routes of Street Art London's tours have changed constantly since their launch in 2011.
"There is art that is meant to last and there is the art that is not meant to last,'' said Australian tourist Zac Kerr as he stood beneath a striking portrait by Portuguese artist Vhils (Alexandre Farto) carved into the wall.
"It's meant to be a short-lived thing, so when you see it, you appreciate it while it's there, before it gets changed by someone else or the government.''
Like New York or Paris, London has become an obligatory stop for graffiti artists from around the world.
Among these pilgrims is Christiaan Nagel, a South African living in London who has peppered the British capital with his sculptures of giant, brightly-coloured mushrooms.
Nagel said he started as a "traditional'' artist, showing his work in gallery exhibitions, but found himself yearning for a wider audience.
"As an artist, you are exhibitionist by nature,'' he said.
"I want my work to be seen everywhere. And the street is endless.''
The graffiti tours have certainly helped to showcase street artists' work.
Nagel said he had received several orders as a result of the tours - but these works will end up away from the streets, in private hands.