France's official hoard of erotica and pornography, lovingly assembled by the Bibliothque Nationale over a period of 170 years, will be thrown open to the startled eyes of the public for the first time this week.

More than 350 books and prints from the forbidden section of the state library officially known as "L'Enfer" (hell) will be presented in an academically meticulous, but often frankly filthy, exhibition in Paris for three months from tomorrow.

The curators of the exhibition consented to give The Independent and its readers an advance peep at their show. Avert your eyes now, or better still, turn the page, if you think that you might be offended.

The material is often beautifully executed, sometimes surreal, sometimes very funny, sometimes brutal. It is steeped in French (and British) history. Much of it is not for the very young or for the faint of heart. The pictures shown here are some of the mildest.

So confident are the French authorities that the exhibition will not offend the French public that a disused Metro station has been taken over as a teaser for the show. From 17 December to 15 January, passengers on Line Ten, between Svres Babylon and Mabillon, will find the abandoned Croix Rouge station turned into an erotic ghost train. Large reproductions of naughty old prints will be glimpsed briefly through drifting curtains.

The "Enfer" section of the Bibliothque Nationale books and prints and photographs purchased, confiscated or donated over almost two centuries is believed to be one of the largest and richest collections of pornographic and erotic materials in the world. The Vatican's secret stash is said to be even larger but that, presumably, will never be opened to the public.

How strong can this stuff be? Given what appears daily on the internet, on cable TV, or in the pages of the Daily Sport, is it possible to be shocked by exquisite, but explicit, 17th-century porn?

The answer is, yes. The exhibition is an eye-opener: a quietly and intelligently displayed but garish cornucopia of sadism, masochism, bestialism, scatology, bums, tits and staring genitalia. It is also a fascinating, and sometimes beautiful, expedition through the dark, winding corridors of the human psyche.

The erotification, or pornification of the internet was inevitable, one learns. Every previous technological or literary advance in human communication from the printing press, to the novel, to the lithograph, to the photograph, to the cinema has been hijacked by the human (or is it mostly a male?) compulsion to meditate, or drool, over our sexuality. Even established writers or artists also, secretly wrote dirty stories or drew dirty pictures out of the act of lovemaking. The exhibition includes examples from the poet Charles Baudelaire and the surrealist artist, Man Ray.

In the DVD/internet age, smut or eroticism is available to all. Most of the material in the exhibition at the Bibliothque Franois Mitterrand on the Left Bank of Paris was originally produced in secret, mostly for an educated, wealthy clientele which pretended to the world that it knew better.

There is one series of explicit but rather funny prints in the exhibition, which consists of innocent sketches of drawing rooms or corridors, with doors which open onto scenes of energetic debauchery. These once belonged to Leon Gambetta, a highly respected late 19th-century, French prime minister.

Typical French hypocrisy? Think again. Some of the most beautifully executed, stylised but brutally explicit prints in the show are from 19th-century Japan.

The exhibition also includes a series of beautiful, early 19th-century, pastoral scenes of haystacks, cornfields and covered rowing boats. If you hold them up to the light, you see eager couples inside the haystacks et al in what the curators of the exhibition coyly call "advantageous positions". This is entitled the "English collection": the prints were all created in Jane Austen's England.

There is also a whole display case devoted to "flagellation novels", which the catalogue describes as an "English speciality" imported to France in the late 19th century.

Most of the material is of French origin. The Marquis de Sade has three display cases to himself, including the hand-written manuscript of "Les Infortunes de la Vertu", composed while he was a prisoner in the Bastille in 1787.

Only bona fide academic researchers have been allowed access to the "L'Enfer" collection until now. The omnipresence of erotic or pornographic images in the modern world has persuaded the French national library that it is permissible, finally, to open the doors of Hell.

"Twenty years ago, such an exhibition would have been unthinkable, certainly one sponsored by a state body such as the Bibliothque Nationale," said Marie-Franoise Quignard, one of the two curators.

"The contents of L'Enfer have been the subject of myth and fantasy for years. People journalists for instance were always pestering us to let them have a look. Attitudes to sexuality and eroticism have changed today. There is a great interest in the connections between literature, art and pornography. The library decided that an exhibition would be acceptable and commercially successful."

The exhibition also explores the history of the Enfer collection itself. Why would a state library gather such works and then hide them away? The same sort of thing exists at the British Library at St Pancras, which has never opened its collection the "Private Case" to general viewing.

The Bibliothque Nationale ... has a statutory duty to collect every book published in France. The torrent of contemporary erotic and pornographic texts do not go into the "Enfer" collection, however. They go to the open shelves of the library.

The Enfer collection consists mostly of works which were published secretly, from the mid 17th century to the 19th century. It also contains a few rare first editions of other erotic works such as Pauline Rage's sado-masochistic classic The History of O, published in 1954.

"The collection was begun in the mid 18th century by the royal librarians," said Raymond-Josu Seckel, the other main curator of the exhibition. "They believed that a national library has a duty to collect everything which could be of cultural or historical interest to scholars in the future."

The royal library became the national library after the Revolution. It created a separate, closed category for sexually explicit materials in 1830. The name "L'Enfer" seems to have been coined some time in the 1840s.

From the beginning, the only outsiders permitted to enter Hell were bona fide scholars who could prove, to the satisfaction of the library management, that they needed to see a particular print or book. Browsing was never allowed.

The collection over 1,700 books and many more prints and pamphlets was obtained partly by raids and confiscations. A large part of L'Enfer came from the private library of a political opponent of the Emperor Napoleon III, who was raided by police looking for anti-Imperial tracts in 1866. They found hundreds of old works which were judged "contrary to good public morals". A court ordered the books to be burned but the then head of the Bibliothque Nationale insisted they should be saved for posterity.

The exhibition reveals some interesting, historical differences in erotic tastes. The earliest, 17th and 18th century, material dwells on the straightforward pleasures of the flesh.

The celebration of the pleasures of pain imposed or submitted begins with the Marquis of Sade in the late 18th century. Pornography from the French Revolutionary period is mostly political, especially scurrilous allegations about the sexual appetite and imagination of Marie Antoinette.

The 19th century concentrates on the blazing sexuality lying below the stern conservative or domestic exterior of life.

"We imposed no particular censorship. Fortunately, there is little in the overall collection which is, for instance, paedophilic," said Marie-Franoise Quignard. What comes over when you go through the whole collection is just how repetitive our sexual imaginations and interests are. You find the same images and themes, the same fixation with male and female genitalia, the same interest in unusual ways of performing acts of sex. In the end you become dulled by it all or you just laugh.

"On the other hand, this is a library, and this is, finally, a literary exhibition. It is fascinating to see how different writers, including well-known conventional writers, like George Bataille, Guillaume Apollinaire, Pierre Louys, adopted different approaches to erotic writing, some desperately serious, some very funny."

Is there anything, finally, that can still shock the custodians of one of the world's most notorious collections of erotica?

"Yes," said M. Seckel. "Most of Sade is elegant and austere in its own way but his 120 days of Sodom is, frankly, disgusting, nothing but shitting and pissing."

"Yes," said Mme Quignard. "There is a photograph of an act of oral sex by the artist Man Ray. Here, it is in the catalogue somewhere..."

She leafed through the pages of the catalogue. We were sitting in the foyer at the main entrance to the library at this stage and attracting some odd looks.

"Here it is," she said, holding up a close-up of an act of fellatio. "For some reason, I find that this goes just too far. Perhaps because it is a photograph and so real, while engravings or lithographs, give you a certain distance and idealisation. Man Ray confronts you point-blank here with something which should, perhaps, better remain intimate."

Much the same might be said of almost everything else in a fascinating but disturbing exhibition, Almost all of this material was, after all, intended for viewing in deep privacy. To view in the sobre surroundings of a national library is educational but deeply unsettling.

The exhibition "L'Enfer de la Bibliotheque, Eros aus secret" is open at the Bibliothque Francois Mitterrand in the 13th arrondissement from 10am to 7pm on Tuesdays to Saturdays and from 1pm to 7pm on Sundays from tomorrow until 2 March. It cost 7 (5) to get in. Under 16s are banned.

For the price of a Metro ticket, however, anyone including under 16s can have a peep into Hell by riding Line 10 from 17 December.

Belfast Telegraph