Observe them, however, and you'll notice faultlines striating this seemingly uniform group: the old-timers who've occupied the same spot for 15 years and the young artists who work their way across Europe in a van; the pastel users and the charcoal wielders; those who fume at the mention of the police, and those who say, licence or no licence, the boys in blue leave them to their own devices; East Europeans and West Europeans; the portraitists versus the caricaturists; those who have friends to tout and conscript and those who work alone.
The street artist seems to have only two gears: the in-your-face chutzpah required to net the customer's attention, and the ardent, mute concentration that kicks in as soon as a tourist hits their stool. The latter is essential, as an audibly judgmental crowd will gather at your back as soon as you put pencil to paper.
London, a Latvian artist called Liga tells me, is the best place to do street portraits: "There's a real buzz here, like nowhere else." In Leicester Square the atmosphere among some 30 artists is one of good-natured competitiveness. August is high-season for street artists, and tourists circle their group like slow-moving tropical fish. The only sign of any artistic temperament is displayed by one or two artists who, when I tell them I'm a journalist, disappear into the crowds, leaving their bulldog-clipped sheets of paper flapping in the breeze.
And their subjects? All tourists. When taken out of one's safe, domestic context, no one is immune to the lure of knowing how others see you.
Cristel Elzenga, 15, from Holland. Picture by Kirke from Sweden. Price, pounds 10
Cristel, visiting her London-based father, is unconvinced by her portrait. She unrolls it and holds it uncertainly between her fingertips. Does she like it? "Ye-es," she says reluctantly, "but it doesn't look like me. It makes me look nicer than I do ... I didn't really want to do it," she flusters. Then why did she? "I don't have any pictures of me now. I wanted to remember what I looked like at 15, and ..." she trails off. And? She shrugs: "And my camera broke."
Mike Kettlewell, 55, from Lincolnshire. PIcture by George from France. Price, pounds 12
Mike tells me the portrait had been an "impulse thing. I'd just been chucked out of the cinema round the corner because the projector broke down, and thought, why not?" He is very pleased with the result: "I was watching people go past and most were nodding their heads, so I think he's got something." When I ask him what he intends to do with it, he says, "I was diagnosed with cancer last year, and while I always want to look on the bright side, I had it done to give to my daughter in case I don't make it."
Costas Georgallides, 61, from Cyprus. Picture by Ishet from Kosovo. Price, pounds 15
Costas, a retired government official, lived in Birmingham as a student over 40 years ago and used to come to London on trips. He is very proud of his home country and before he will discuss his portrait, he forces a promise out of me that I'll visit him, handing me photographs of his villa. His portrait today is "just accidental. My brother-in-law wanted to have one and I came along too. I think it is very, very good; a bit bright perhaps, but very three-dimensional. I am satisfied." He's not sure what he'll do with his new pastel image; he has written books of poetry and short stories, and it might end up as his author picture.
Jimin Kim, 34, and her daughter, 8 months, from Korea. Picture by Liga from Latvia. Price, pounds 5 per face
Jimin speaks no English so her sister translates. Jimin's husband works in Belfast and she, her sister and her baby daughter are over in Britain for a short break. The two sisters are delighted with the caricature: "It's very funny. It will always make me laugh. It is a good memento of London." The picture will be framed and hung in the Kim living room in Seoul. "I wanted it for my baby," Jimin says, "for when she grows up, so that one day she will come back to London and to Leicester Square."
Khalifa Mustapha, 17, from Atlanta, USA. Picture by Ferdinand from Albania. Price, pounds 15
Khalifa is on a long holiday to visit his cousins who live in Woolwich. His 17th birthday fell in the middle of his trip. "We were all walking past here a few weeks ago and we thought it looked pretty nice. My cousin told me I should do it for my birthday. She gave me the money." He was "kind of scared" to come on his own so brought his younger cousin Anso with him for moral support. So what does he think of it? "Beautiful! It's going straight on my bedroom wall when I get back to the States."
Becky Harmer, 15, from the Isle of Man. Picture by an artist who ran when asked his name. Price, pounds 15
"My mum pushed me into it," Becky tells me. "I do like it, though, because it doesn't really look like me." Becky believes her mother will "frame it and stick it on the wall". Will she mind? "I'm hoping that it will replace an awful one she has up at the moment of me, naked. That's why I agreed to have it done." When her mother appears, she tells a different story: "Becky's an artist herself. There's nothing like a bit of street art to encourage her."