It is possible that these are just the overzealeous jobless gangs however I will not rule out the possibility that some OBL's AQ cells are changing their strategy and trying to engage Europe in economic stability through violent protests. The immigrants probably know that how much effective these strategies are in derailing economic activity in their home countries.
"While the demonstrations have been orderly and peaceful, groups of 200 to 300 youths who police say do not appear to be participating in the organized marches have appeared suddenly during concluding rallies, taunting police and creating havoc.
Police have speculated that the gangs may be from the poor suburban areas that erupted in riots last fall. In those disturbances, youths across France -- many of them immigrants or French-born children of immigrants -- burned thousands of cars and hundreds of public buildings and private businesses to protest government indifference to the joblessness and lack of social services in their communities. Little of that violence spilled over into Paris or other urban centers."
I must say that French society and government must act with caution as they are already trying to recover from last years scars caused by riots. As a student of Economics I must say that the government should treat young and adult workers alike. Most of the Europe's job market problems are owed to strong labor unions. The inside workers always make it difficult for outside workers to enter into job market as they see it as a threat. I wish I had time to explain the economic theory of inside and outside workers and the interaction of European labor unions but I am sure you can find a lot on internet.
Related: French Qurater I; French Quarter II.