Rioting by youths in a Paris suburb has highlighted the discontent among sections of France's immigrant population.
The BBC News website's Henri Astier explores the sense of alienation felt by many French Muslims.
Like many others, Nadir Dendoune's suburb has turned into a ghetto
When Nadir Dendoune was growing up in the 1980s, his home town of L'Ile Saint-Denis, north of Paris, was a fairly diverse place.
"We were all poor, but there were French people, East Europeans, as well as blacks and Arabs," says Mr Dendoune, 33, an author and something of a celebrity in his estate.
Two decades on, the complexion of the place has changed.
"On my class photos more than half the kids were white," he says. "On today's pictures only one or two are."
L'Ile St-Denis is among the "suburbs" around French cities where immigrants, notably from former North African colonies, have been housed since the 1960s.
Blighted by bad schools and endemic unemployment, the suburbs are hard to escape.
The immigrants' children and grandchildren are still stuck there - an angry underclass that is increasingly identified through religion.
Ten years ago these youths were seen as French "Arabs".
Now most are commonly referred to, and define themselves, as "Muslims".
Many countries have ethnic and religious enclaves. But in France they cause particular alarm, for three reasons.
First, they are not supposed to exist in a nation that views itself as indivisible, and able to assimilate its diverse components.
Separatism, the French are told, is a plague afflicting the Anglo-Saxon multicultural model.
The government bans official statistics based on ethnicity or religion. As a result, no one knows exactly how many Muslims live in the country - at least five million is the best guess.
Second largest religion
Five million Muslims (estimate)
35% Algerian origin (estimate)
25% Moroccan origin (estimate)
10% Tunisian origin (estimate)
Concentrated in poor suburbs of Paris, Lille, Lyon, Marseille and other cities
Ghettos also threaten another tenet of French identity - secularism.
As the country celebrates the centenary of the separation of Church and State, Islam is seen as the biggest challenge to the country's secular model in the past 100 years.
Thirdly, the worldwide rise of Islamic militancy strikes fear in the heart of a country that is home to Europe's biggest Muslim community.
French police know that there is no shortage of potential jihadis in the country.
The assertiveness of French Islam is seen as a threat not just to the values of the republic, but to its very security.