By Caroline Wyatt
BBC News, Paris
As darkness fell at the Place de La Republique, the acrid smell of tear gas drifted through the early evening air.
Most of the rioters were aged between 15 and 25
It mingled with a distinct smell of cannabis as the shouts and jeers of the crowd echoed around this historic square in what has become a Paris springtime of discontent.
These, though, were not the peaceful marchers of earlier in the day but youths who had come intent on violence, seeking running battles with police.
Earlier in the afternoon, hundreds of thousands of protesters had gathered at Place d'Italie in the south of the city.
But the festival atmosphere quickly darkened as a small group of troublemakers began to target trade union stewards trying to keep order on the march.
Those youths smashed the windows of a cafe and soon took on the trade unions' security men in random brawls, as the peaceful protesters - young and old - continued on their route across the River Seine and on to Place de la Republique.
A sudden downpour failed to quell the protesters' spirits as they marched, chanting slogans against the government and its new jobs contract.
But once they reached their destination in the eastern city centre, most student demonstrators - and the workers who came out in support of them - quickly left as more youths descended on the square, with the sole aim of picking a fight with the riot police.
These youngsters - aged between 15 and 25 - were wearing hooded tops that obscured their features. Many wore masks, so they could not be identified.
Some had soaked scarves in lemon juice to wrap around their mouths before they came, to counteract the effects of the tear gas they knew would be unleashed by police.
According to the riot police, many of these youths had come in from the suburbs outside Paris looking for trouble: this violence seen by some as a continuation in Paris city centre of what began in November's riots in the suburbs.
And the riot police were ready and waiting for it all.
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This, after all, has become the ritual ending to almost every demonstration here in Paris, when the protesters go home and the troublemakers, known as "casseurs", or wreckers, move in for the fight.
The interior ministry put 4,000 riot police on the streets of Paris in readiness, after similar scenes at Les Invalides last week marred the end of demonstrations.
Wearing full riot protection kit, their riot shields held aloft, the CRS police stood patiently as the first bottle was thrown at them, smashing harmlessly on a shield.
4,000 riot police were on the streets of the capital
Then a paving stone was hurled at them. More bottles. And finally, anything that came to hand, the police advancing and retreating in waves as one group of youths banged on a drum, adding a rhythmic bass to the shrill of police and ambulance sirens that pierced the air.
Ambulances arrived at the square, taking away some injured protesters, just as a jeering, chanting crowd of youths tried a mini charge at the police.
Soon, running skirmishes broke out across the square between rioters and police officers who had also broken up into smaller groups to deal with them.
By 1900 local time, just as the sun emerged once again from the clouds, the Place de la Republique had turned into a combat arena, the scene of almost ritualised violence.
"This is a shame," said Thierry, one of the student demonstrators, as he left the square to avoid the tear gas, just as police were bringing out the water cannon to disperse the crowds.
"This is all the people see, and it is the wrong image of French youth. This is not us. And this is not what we want," he said.
Yet this is the image that will remain with many, the image of a France in chaos and disorder: a France that is failing to offer hope to an increasingly nihilistic youth.
The skirmishes may be small in scale and number - perhaps 200 violent youths in all - but they fill the television screens at home and abroad, week after week, damaging France's image and putting further pressure on the French government to act.
The irony is that the government's new job contracts were brought in hurriedly to help youngsters in France's troubled suburbs - where last November's rioting brought severe social and racial problems into sharp focus.
On the worst of the housing estates, joblessness among the young, especially those whose families are of Arab or north African origin, is about 40%.
Yet Prime Minister Dominique de Villepin's answer to that problem is not working, as growing disorder on the streets adds fresh urgency to the search for a political solution.
These scenes of almost ritualised violence are becoming wearily familiar to many in France.
All this adds to the intense pressure on the French prime minister to act.
He has said he is determined to tough it out, where his predecessors have failed.
But facing the anger of the streets, and a divided cabinet behind him, that could prove the greatest challenge of his political career.