I have not personally done time in France, but get on the wrong side of a magistrate's hunch, and it is quite easy to win that privi-lege. The arbitrariness of France's justice system has always rather disturbed its citizens, as it is perhaps designed to do. At present, it positively scares them. This much is clear from the high ratings for televised sessions of a public inquest into their latest cause for anguish. Rub your eyes and you may well catch Robespierre, the master of the Terror, back in business.

The parliamentary inquiry, which opened in January and runs until June, arose out of a stupefying miscarriage of justice in a bleak corner of northern France, which, as it happens, is Robespierre's homeland. Now it looks as though the whole creaking system of French justice is up for revolution.

The principal figure in the furore over the case of a suspected paedophile ring in Outreau, near the port of Boulogne, is a young investigating judge fresh out of France's training academy for magistrates. Still in his twenties when he first moved into action, the fledgling judge put 18 people behind bars without trial for up to three years while pursuing his intuition - there was, he acknowledges, no material evidence - that all were guilty of child abuse. This was the time of a media frenzy over paedophile horrors, when the popular uproar among the French was almost as loud as that whipped up by the tabloids in Britain. French papers published names and faces of the supposed monsters.

Almost all the Outreau suspects insisted they were innocent, including one who committed suicide in detention. And when they at last got their day in court, 13 of the remaining 17 were cleared outright, prompting President Jacques Chirac to extend to them the nation's apologies. As well he might. For what the nation has witnessed on television is a nightmare of innocent lives in ruins, marriages gratuitously wrecked and bewildered children taken into care. It all started with one of the four people who were eventually convicted tossing the innocents' names to the prosecution at random to put it off the scent.

This is enough to scare anyone. The popular reaction in France is: "If it could happen to them, it could happen to us." A democratic country's system of justice is much like its public health system: if the people have faith in it, it works; if they don't, it doesn't. Here the French, as in other things, are rather the opposite of us British. They actually like their generous and accessible healthcare. Before the law, however - and this despite Tony Blair's scrape at established liberties - they have more reason than we do to doubt that an accused will receive a fair trial. Indeed, France's prosecutor general ruefully admitted just this month that French justice has acquired an image of doing actual harm.

The presumption of innocence until guilt is proven gets short shrift. The principle exists, but not the mechanisms to uphold it. "Murderer caught" is the classic newspaper headline when a suspect is detained for questioning. Most of the Outreau defendants ultimately had the assistance of a defence lawyer, though they found it a waste of time. "I always wondered what my lawyer was for," said one of those acquitted, a worker priest. The lawyers said they weren't allowed to do their jobs.

The judge in question, Fabrice Burgaud, was a product of the Ecole Nationale de la Magistrature whom MPs on the inquiry referred to as "just a kid". Their view is that he was too inexperienced to hold responsibility for taking away his fellow citizens' freedom. French justice is manned mainly by officials who obtain their robes not by gaining seasoned experience of the law in action, but by studying for a diploma at a school from which they graduate, after a three-year course, at the age of roughly 27.

Burgaud, now 34, appeared before the inquiry commission with his reputation already drawn by the suspects acquitted: zealous, arrogant, deaf to testimony that conflicted with his convictions. There was more than a touch of Robespierre - the revolutionary dispenser of soulless justice, "the Incorruptible" - to their portrait, as there was to Burgaud's performance before the commission. Pale and tense, he insisted he had followed procedure to the letter. Nor had his judicial superiors once suggested he was on a false track, or that he might be exceeding his powers, which go straight back to revolutionary times.

The inquiry has no remit to disqualify Burgaud or any other judicial figure: only to establish what has gone wrong and to propose what now promises to be a major overhaul. Burgaud emerges as a living symbol of the system's flaws. For it is his role of juge d'instruction - investigating magistrate - that causes the sharpest controversy. This is a very Latin post, created by Napoleon Bonaparte, who described its occupant as "the most powerful man in France".

The task of this state functionary - neither police detective nor occupant of the bench, but a criminal umpire of sorts - is to make a suspect confess prior to trial. To obtain a confession he (or increasingly, these days, she) can imprison a suspect for months, years even, without trial. Never mind hard evidence: "intimate conviction" of guilt is sufficient to keep a suspect behind bars. The concept is rooted in the medieval Catholic Inquisition, in which confession carried more weight than proof.

In the wake of the Outreau affair, the call has been grow-ing among politicians and legal professionals for a move towards the Anglo-Saxon model, where the burden of proof is on the police, who must nail down guilt with factual evidence, and where lawyers for the defence have the same rights as prosecutors.

The absence of habeas corpus guarantees might be a little less intimidating if police detention cells and jails that house suspects on an investigating judge's whim were any less archaic than the system itself. In February, a Council of Europe human-rights report branded detention conditions in France so lamentable that they appeared to be "from another age". The Chirac government has said modernisation of prison conditions is under way, but it admits that change will not happen until 2007.

The fairness of the justice system similarly came under scrutiny during the 1990s, when a posse of high-flying juges d'instruction won renown by taking on corruption in politics and big business. For a time, they were heroes and heroines. But moods change. The message of the latest film by the director Claude Chabrol, L'Ivresse du pouvoir (Drunk on Power), just out in Paris, is that shady executives in state industry pose less harm to society than their overzealous huntress: an investigating magistrate.

It is true that parliament did bring in reforms only three years ago to bolster defendants' rights, mainly by limiting the powers of judges such as the huntress. A new breed of magistrate was created - the "liberty judge" - to take the place of the investigating judge in deciding whether suspects should be detained or stay free while under investigation. Defence lawyers also won leave to be at their clients' sides from the moment the police hold them for questioning, even before a juge d'instruction becomes involved. Alas, the news from the legal underground is that none of this has made much difference. There aren't enough liberty judges to do more than follow the wishes of their investigating colleagues, and improved support for suspects is honoured in the breach.

The likely outcome of the outcry is, indeed, a stronger swing towards the Anglo-Saxon model and the presumption of innocence. Fixing the problem of the judicial maturity gap is also a priority. All the same, the French are wary of granting the defence the sort of power our system provides: too often, they conclude, it leads to the rich going free and the poor going down. This violates equality, French companion of fraternity and that dented principle, liberty. Robespierre won't give ground easily.

David Lawday's biography of Talleyrand, Napoleon's Master, is published by Jonathan Cape in September