Rioting by youths in a Paris suburb has highlighted the discontent among sections of France's immigrant population.
Like many others, Nadir Dendoune's suburb has turned into a ghetto
The BBC News website's Henri Astier explores the sense of alienation felt by many French Muslims.
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.
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
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.
Most French Muslims say Islam is compatible with French values
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.
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 Western 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.
A different view
Is such alarm justified? The view from the suburbs invites a nuanced, and ultimately sanguine, assessment.
Some groups do advocate cultural separation for Muslims - but they do not speak for many.
Far more common is the attitude of Nour-eddine Skiker, a youth worker near Paris: "I feel completely French. I will do everything for this country, which is mine."
Mr Skiker's Moroccan origins mean a lot to him. But, like many youths in the suburbs, he sees no contradiction between being French and having foreign roots.
The main problem is that many French people do, says writer Nadir Dendoune.
"How am I supposed to feel French when people always describe me as a Frenchman of Algerian origin? I was born here. I am French. How many generations does it take to stop mentioning my origin?"
And crucially, the suburbs are full of people desperate to integrate into the wider society.
"I do not know a single youth in my estate who does not want to leave," Mr Dendoune says.
France's Muslim ghettos, in short, are not hotbeds of separatism. Neither do they represent a clear challenge to secularism - a doctrine all national Muslim groups profess to support.
Immigrants have been housed in estates around French cities
"We have no problem with secularism," says Lhaj Thami Breze, president of the Union of Islamic Organisations of France (UOIF).
He argues that by establishing state neutrality in religious matters, the doctrine allows all religions to blossom.
Islam has adapted to local laws - from Indonesia to Senegal - and is adapting to France, says Azzedine Gaci, who heads the regional Muslim council in Lyon.
This is not just the leaders' view. A 2004 poll suggested that 68% of French Muslims regarded the separation of religion and state as "important", and 93% felt the same about republican values.
All observers agree that jihadism does pose a direct threat to the country.
The fact that - in France as elsewhere - the militants speak for a tiny minority of Muslims does not make the threat less severe.
But as Islam expert Olivier Roy notes, bombers should not be seen as the vanguard of the Muslim people. Jihadis everywhere, he says, are rebelling both against the West and their own communities.
The great majority of Muslims resent the extremists in their midst - although many in France do not recognise this.
Yazid Sabeg, an industrialist and writer, says the French have "a real problem" with both Arabs and Islam and equate both with extremism.
The most worrying aspect of the separation between French Muslims and the rest of society is that it breeds suspicion on both sides.
"We must tell youths that France does not want to hold them down," says Rachid Hamoudi, director of the Lille mosque in northern France.
"We must ensure that the community trusts its country, and vice-versa. If you get to know me, you will get to trust me. If I get to know you, I will trust you."