An javelin thrower stands poised to make his throw -- but in place of a spear, he holds a missile.

The striking image, stencilled in black onto a dirty wall, is one of two new works recently unveiled on the website of British satirical street artist Banksy to coincide with the London Olympics.

The other shows a pole-vaulter, springing over a rusty fence onto an abandoned mattress.

Banksy -- whose works sell for hundreds of thousands of dollars -- is famed for his secrecy, but his witty observations about British life can usually be spotted on the street.

The location of the two new stencils, however, has been kept under wraps -- prompting speculation that Banksy feared they could be removed as part of a reported drive to clean graffiti from London's walls for the Olympics.

British Transport Police arrested four graffiti artists this month and banned them from Olympic venues in an apparent bid to prevent the Games becoming a target for subversive spray-painters.

"There's a more widespread clean-up operation than normal, definitely," street artist Mau Mau told AFP. "There's loads of work disappearing from the tracksides by the railways."

Mau Mau, who declined to give his real name, made his own Olympic offering to the city walls a few weeks ago: an overweight McDonalds clown, carrying a torch labelled "Coca-Cola" that was spewing out a thick plume of black smoke.

Both brands are official sponsors of the Games.

"I painted it to protest against the corporate takeover of the Olympics," he explained.

Mau Mau created the west London mural with the permission of the wall's owner, but it was whitewashed just six days later by the local council, which said it had received a complaint from a member of the public.

"It wasn't offensive," said Mau Mau. "It was just a picture of a fat clown."

The artist believes he may have fallen foul of strict regulations designed to protect the Games' corporate sponsors.

His mural clearly featured the Olympic rings, a symbol that can only be used with the approval of the International Olympic Committee.

With the rings emblazoned on his vest, Banksy's javelin-thrower also breaks the rules, which could give Games officials additional cause to paint over it -- if they can find it.

James Cochran, whose vivid mural of the world's fastest man Usain Bolt beams over an east London car park, said he was careful to seek official approval before shaking the first aerosol can.

"They said, "By the way, don't touch the Olympic rings -- there's very strict copyright control on that," he recalled.

"They're getting a bit authoritarian," the British-Australian artist told AFP. "The Olympic Committee have got to be careful that they don't infringe on freedom of expression. Sometimes you think, 'Jeez, Beijing was more open-minded.'"

The Games' corporate sponsors, and the lengths to which organisers have gone to accommodate them, have become popular targets of Britain's vibrant underground art scene, particularly near the Olympic Park in east London.

At Hackney Wick station on the edge of the park, someone scrawled the word "Shame" in huge letters over a mural commissioned by Coca-Cola, forcing the company to have the work painted over.

For Cochran, the Games are an opportunity to spread a more cheerful message. His image of Bolt, with brightly coloured rays pulsing from his face in all directions, is his "homage to the buzz around the Olympics".

"I love London, and I see this as an amazing and positive time for the city," the 39-year-old said.

"When the Olympics are done and everyone's gone home, it would be nice to have something that speaks about that time. But who knows if it will last? That's the interesting thing about street art -- anything can happen to it."