THE authorities are taking no chances. This morning, every train station linking the area west of Paris to the centre of town will be patrolled by tough CRS riot policemen on the lookout for young men wearing the "gangsta" uniform of hooded sweatshirt, reversed baseball cap and spinnaker-sized jeans.
The barbarians have been at the gates of the capital for years, penned in bleak housing estates in towns with incongruously bucolic names like Chanteloup-les-Vignes and Mantes-la-Jolie.
Last week, to the horror of the city's bourgeois inhabitants, they breached the stockade. Families in the huge shopping centre in La Defense on the western edge of Paris watched in fear and fascination as 300 youths swarmed on to the modernistic plaza and started laying into each other with baseball bats.
As they loped through the terrified crowds they chanted "Sept-huit! Sept-huit!" The shoppers knew what it meant. It's the number of the department of Yvelines, in the badlands beyond the Peripherique where the youths normally spend their days, out of sight and out of mind. The CRS had been tipped off about a possible riot and intervened to stop what looked like an inevitable bloodbath.
But the symbolism of the event was unmistakable. To the French middle class the event seemed as significant as the Christmas morning in 406 when the Rhine froze over and the Roman legions on the western bank saw the Vandals shuffling towards them and knew it was all over.
Fights between rival gangs are nothing new in France. Until now, however, the participants have been considerate enough to conduct them in the privacy of their own ghettoes. For that is what places like Chanteloup and Mantes, where the rival gangs hailed from, have become.
Many of the inhabitants are "Beurs", immigrants from North Africa. In the peculiar grading system of French racism they figure at the top. It is Arabs rather than blacks that supporters of Jean-Marie Le Pen and Bruno Megret reserve their hatred for.
They live lives of monotony and frustration, frequently unemployed, devoid of the stimuli of normal urban dwelling. They would have difficulty relating to the image of France projected by its leaders - a harmonious society enjoying the fruits of a booming economy - and live screened away from the world in big housing projects - cites - that ring most big towns in France.
When things are quiet the euphemism for these places is quartiers sensibles - sensitive areas. When things turn ugly they become quartiers chauds. Most French people have never visited these places and have no desire to. The council blocks of Chanteloup are tucked away in one of the richest areas of France, not far from well-upholstered suburbs like Versailles and St Germain-en-Laye to the west of Paris.
Of the municipality's 9,000 inhabitants, 7,000 live in council flats. The area boasts 73 ethnic groups. The majority are North Africans, many of whom were born in France or taken there shortly after birth. The unemployment rate is 18 per cent - eight points higher than the national average.
Council officials say there is little tension between whites and the other inhabitants. The teenage gangsters of the estates work off their excess energy on young men of the same background and class as themselves and who live in the same concrete boxes - but in Mantes, a short train ride down the Seine valley. More than half of the 44,000 inhabitants of Mantes live in Val-Fouree, an outcrop of high-rises towering over windswept concrete plains.
Local chroniclers date the enmity to 1990, when youths from Chanteloup set about some Mantes schoolboys on a train. Word got to the adolescents of Val-Fouree, who speedily mounted a punitive expedition. The clashes have gone on ever since. The fuss created by the incident of La Defense have revealed a number of uncomfortable truths about life in the cites.
In many estates around France, the weekend punch-up between rival gangs, often in shopping centres, is a regular fixture, organised along the lines of a football league. The clash at La Defense was arranged in advance by the group leaders.
A Mantes town hall official said: "There's a sort of clan culture. It's like following a football team with your own clan, colours and leaders. It involves only five per cent of everyone in the cites, but they make it rotten for everyone else."
The gang members are getting younger and the weapons more serious. Butchers' knives and meat cleavers were found on some of those arrested at La Defense. The police, never too keen to venture into the estates, say they haven't got the numbers to intervene effectively. They are facing an explosion of crime, much of it violent and carried out by youths.
In a typical incident last week in the Val-d'Oise north of Paris, 10 young men raided a secondary school after a portable phone was stolen. In the resulting fight shots were exchanged and a 15-year-old was wounded in the leg.
Large numbers of sociologists have been employed to try to find the causes of the aggression and there have been many initiatives to try to persuade the adolescents to do something better with their lives. They have not worked. Yazid Kherfi is a consultant on urban violence who speaks from experience. He used to be a young thug himself in Mantes.
Things are worse now than they were then, he believes. He said: "The young no longer believe in collective action. The sickness goes very deep. It's much worse than in the Eighties, but it's perhaps less visible now." The La Defense incident would appear to have changed that. But why have the gangs decided to take the trouble to town?
One explanation is that they are raising the stakes in a campaign to shock the bourgeoises. It is a regular weekend ritual for the young inhabitants of the cites to take the train to Paris and spend the day hanging out in one of a number of well-defined spots.
A favourite is the broad plaza at Les Halles in the middle of town. Parisians give a wide berth to the main walkway, where powerfully built young men smoke joints, leer at the girls and glare at other males.
It's an unnerving sight, but it is possible to feel sympathy as well as alarm. Behind the bantam cock posturing is a burning desire to be noticed, to be taken seriously. A young English woman who lives off the square said: "The message is we're here, we exist, we are not going to go away."
That would seem to be the signal intended by the youths of Mantes and Chanteloup when they chose one of the city's main cathedrals of consumerism to mount their gladiatorial contest.
Thanks to the CRS the fight was never resolved. The authorities are convinced that it has been postponed rather than cancelled, hence the doubling of the watch on the capital's gateways this weekend. The boys of Mantes and Chanteloup will have to have their scrap elsewhere this time. But they'll be back.
31 January 2001: Curb on teenage crime after Battle of La Defense
6 December 2000: Teen killings trend makes France aware of 'sink estates' crime
27 January 2000: French effort to stamp out classroom violence
17 May 1998: French face summer of ghetto gang wars
6 April 1998: French schools to reopen but the violence remains