The ancient capital of the Khmer empire is one of the archaeological masterpieces of the world and the spiritual and cultural heart of Cambodia. It is also the victim of a slow and painstaking rape, an assault mounted with chisels and drills by impoverished locals for the benefit of wealthy collectors in the West.

Hidden by the bamboo shoots on the approach to the little-used northern gate, an intricately carved bas-relief of sandstone lotus leaves decorates a section of the wall bordering the path. Lying in front of it yesterday were a cluster of motorcycles hastily thrown down by a gang of five men who vaulted the wall and disappeared on hearing the approach of footsteps.

In each of the more than 50 niches beautifully carved into the stone there was nothing left but scarred rock. The tell-tale signs of chiselling were all that remained where once a cross-legged Buddha would have sat. Elsewhere, carved panels had tumbled onto the jungle floor, heads had been clumsily hacked off and only amputated legs were visible where the torso of a goddess had been hauled away.

The Khmer buddhas are by now on their way to the antique shops of Bangkok, or the auction houses of the West. They are bound ultimately for secret collections of Angkorian art in an illicit and thriving trade that is consigning one of the wonders of the world to a slow death.

"It is tragic. And criminal," said Anabel Ford, a Californian archaeologist on a project to photograph the spring equinox at Angkor. "There is no way to know if it happened 20 years ago or 20 minutes ago, but there is no mistake that beautiful pieces are missing."

The temples of Angkor were built between the 9th and 14th centuries, when Khmer civilisation was at its height and the empire stretched north to Yunnan in China and from Vietnam westwards to the Bay of Bengal. Unparalleled in south-east Asia, they are a living testament to the extraordinary creativity of the Khmer.

The writer and broadcaster Dan Cruikshank became increasingly concerned at the situation in Angkor during the filming of Around the World in 80 Treasures. He said he was immediately struck by the vulnerability of the more than 100 temples stretched over 77 square miles of dense jungle.

"Most of it is not guarded or policed. Even at Angkor Wat itself it is very easy to pick something up. There is a lot still unexcavated and you can just pick it up. It's a gigantic problem. It would be easy if one were determined to knock off the odd head. There's no one around but the occasional tourist."

Cruickshank says the Western collectors are as much to blame for the looting as the gangs that bring the stone saws to bear on the priceless carvings. The Khmer treasures are, he says, a victim of their own beauty.

"The quality of the 12th-century workmanship is outstanding. There is a tremendous fusion of Hindu and Buddhist art with Indonesian sensibility. There's nothing else like it and it's a loss to mankind at large."

The rape of Angkor is not strictly a modern phenomenon and looting has been under way since the complex was abandoned in the 16th century and the court moved to Phnom Penh.

The destruction increased with the rise of the Khmer Rouge. In 1971 Pol Pot's guerrillas moved into Angkor Wat, lit fires in the galleries, installed rocket launchers and started slicing the heads off sculptures. They sold their bounty across the border into Thailand to help finance the war effort. But it was after the fall of the Khmer Rouge that the pace of looting really picked up. Without the guerrillas to ward off potential thieves, the rape began in earnest.

An attempt to move up to 7,000 of the most valuable pieces to the conservation office in the nearby town of Siem Reap in the early 1990s merely exacerbated the conflict. In February 1993 thieves wielding machine-guns and rocket launchers attacked, killing one guard; they left with 11 of the most valuable statues.

Cambodian officials say they are doing all they can. They claim that the presence of squads of monument rangers clutching walkie-talkies and checking all visitors for their $20 per day photo passes has helped to dampen the looting spree. But conservation officials admit they are fighting an uphill battle.

"Vandalism has multiplied at a phenomenal rate," the conservation agency said in a recent statement, with thieves "employing local populations to carry out the actual thefts. Heavily armed intermediaries transport objects, often in tanks or armoured personnel carriers."

Today Angkorian antiquities can be browsed in air-conditioned comfort in Bangkok's River City complex or Singapore's Tanglin shopping centre. There are even persistent rumours of the existence of a catalogue containing detailed photographs of Angkorian statues and bas-reliefs, allowing wealthy Westerners to order items still in-situ.

Three countries still allow antiquities to be purchased without documentation: Australia, Japan and Switzerland. On the black market, a life-sized Buddha from Angkor can fetch at least $250,000.

Roland Eng, a former US ambassador to Cambodia who recently returned, said two severe factors ensured that the looting continued: the parlous state of the economy and the exorbitant prices that Angkorian art fetched on the international market. "The country remains very poor. The army is very poor," he told The New York Times. "We have to encourage people not to buy antiques when they cannot trace the source."

Growing cultural awareness and tighter controls - such as the American ban on trade in illicit Cambodian artefacts passed in 2003 - hold out some hope that future generations will see the wonders left in the jungle by the Khmer kings. But the overwhelming sentiment is one of sadness at the irreparable damage which Cambodians have been driven by poverty and the greed of foreigners to inflict on their own heritage.

King Norodom Sihanouk, who abdicated in favour of his son last year, contrasts the ancient glory with the modern theft of anything that might have a market: "It is very sad the Cambodian people were so masterful and skilful but now they plunder their own history."