In what may prove a particularly incendiary cable, US diplomats describe a world of sex, drugs and rock'n'roll behind the official pieties of Saudi Arabian royalty.

Jeddah consulate officials described an underground Halloween party, thrown last year by a member of the royal family, which broke all the country's Islamic taboos. Liquor and prostitutes were present in abundance, according to leaked dispatches, behind the heavily-guarded villa gates.

The party was thrown by a wealthy prince from the large Al-Thunayan family. The diplomats said his identity should be kept secret. A US energy drinks company also put up some of the finance.

"Alcohol, though strictly prohibited by Saudi law and custom, was plentiful at the party's well-stocked bar. The hired Filipino bartenders served a cocktail punch using sadiqi, a locally-made moonshine," the cable said. "It was also learned through word-of-mouth that a number of the guests were in fact 'working girls', not uncommon for such parties."

The dispatch from the US partygoers, signed off by the consul in Jeddah, Martin Quinn, added: "Though not witnessed directly at this event, cocaine and hashish use is common in these social circles."

The underground party scene is "thriving and throbbing" in Saudi Arabia thanks to the protection of Saudi royalty, the dispatch said. But it is only available behind closed doors and for the very rich.

More than 150 Saudi men and women, most in their 20s and 30s, were at the party. The patronage of royalty meant the feared religious police kept a distance. Admission was controlled through a strict guest list. "The scene resembled a nightclub anywhere outside the kingdom: plentiful alcohol, young couples dancing, a DJ at the turntables and everyone in costume."

The dispatch said the bar featured a top shelf of well-known brands of liquor, the original contents reportedly replaced with sadiqi. On the black market, they reported, a bottle of Smirnoff vodka can cost 1,500 riyals (£250) compared with 100 riyals (£16) for the locally-made vodka.

In a venture into Saudi sociology, the diplomats explained why they thought their host was so attached to Nigerian bodyguards, some of whom were working on the door. "Most of the prince's security forces were young Nigerian men. It is common practice for Saudi princes to grow up with hired bodyguards from Nigeria or other African nations who are of similar age and who remain with the prince well into adulthood. The lifetime spent together creates an intense bond of loyalty"

The cable claimed it was easy for would-be partygoers to find a patron out of more than 10,000 princes in the kingdom. Some are "royal highnesses" with direct descent from King Abdul Aziz, while others are "highnesses" from less direct branches.

One young Saudi told the diplomat that big parties were a recent trend. Even a few years ago, he said, the only weekend activity was "dating" among small groups who met inside the homes of the rich. Some of the more opulent houses in Jeddah feature basement bars, discos and clubs. One high-society Saudi said: "The increased conservatism of our society over these past years has only moved social interaction to the inside of people's homes."