Nicolas Michel stopped playing the bagpipes to answer his mobile phone. Trouble ahead, friends told him. Don't go up the boulevard de la Bastille, writes Lara Marlowe in Paris.
Michel sniffed the air like a giraffe. "Tear gas," he said.
A wave of panic shot through the crowd, prompting a mini-stampede on the Austerlitz bridge. "I'm getting out of here," said the 28-year-old unemployed carpenter.
Some 200 casseurs (as troublemakers from the immigrant suburbs are called) were attacking marchers to steal their mobile telephones.
"I called in the police," a volunteer security guard for the communist CGT union told me later. "About 10 people were beaten up. If the police hadn't rushed in, it would have been real merde. These people are hurting our cause."
For more than five hours, as the procession advanced towards the Place de la République, the mood shifted between festive, angry and fearful. At the bottom of the boulevard de la Bastille, I saw helmeted CRS (riot police) charge into the crowd.
At la République police fired "paint-bullets" which spray who skirmished with them. It took water cannon to finally disperse them after sunset.
Steve Pantu (18) darted out from one melee with the CRS. The son of immigrants from Cuba and the former Zaire, he was dressed in black from his runners to his "hoodie" and seemed to be enjoying himself immensely. "The casseurs who rob demonstrators are not good," he said. "But those who attack the CRS are good."
Pantu, who is enrolled at a maintenance school, insisted he had not come from the suburb of Cergy-Pontoise looking for a fight but to protest against prime minister Dominique de Villepin's First Job Contract (CPE). "It won't change unemployment in the banlieue," he insisted.
Youth unemployment runs up to 50 per cent in the suburbs. Mr de Villepin claims freedom to fire employees under age 26 without justification during a two-year trial period would ease the problem.
"Chirac, Villepin, Sarkozy: Your trial period is over," the demonstrators chanted, referring to the president, prime minister and interior minister.
A graphics student from the University of Paris smoked a joint while photographing what he called the "spectacle" of CRS in battle gear. They in turn photographed him.
By one count, France has endured 24 "social movements" in the past four years.
"We're prone to this sort of thing," a police commissioner in charge of a CRS unit sighed. "Personally, I think it's a waste. We should let the majority carry out their plans, and if we don't like them vote against them next time. What's happening here is the blackmail of the street."