The change has been significant over the last five years, Mr. Geisser said. “Now we’re at a crossroads,” he said, of a complicated European anxiety that stems from economic crisis; the fear of globalization; the perceived increase in immigration as European birthrates fall; and the subsuming of national states into an enlarged Europe.

“There is an angst over identity in Europe,” he said. “There’s a feeling that Europe is becoming smaller and less important. Europe is like an old lady, who whenever she hears a noise thinks it’s a burglary.” This generalized anxiety and fear is translated into a specific one, he argues: Islam, “a box in which everyone expresses their fears.”

The European Union is believed to have more than 15 million Muslims and perhaps as many as 20 million. France has five million to six million Muslims, the most in Western Europe.

In general, relations between Muslims and other Europeans have been good. But the terrorism associated with attacks in France in 1995 and 2001 in the United States has resonated through the years, reinforced by the Madrid train bombings in 2004; the killing that year of the Dutch film director Theo van Gogh, a critic of conservative Muslims; the London bombings of 2005; and the controversy over Danish cartoons of the Prophet Muhammad published the same year.

In 2004, France banned the head scarf (and other signs of religious affiliation) in public schools. It is now debating a ban on the burqa, by which the government seems to mean any full facial covering, including the niqab, which shows the eyes. That controversial measure is caught up in a government-sponsored debate over national identity, led by the ministry that also handles immigration.

Image AT PRAYER Muslim worshipers prayed in a mosque earlier this month in Marseille, France. The city is at least 25 percent Muslim. Credit... Michel Gangne/Agence France-Presse

Both measures have been widely criticized as political maneuvers by President Nicolas Sarkozy, capitalizing on social fears to unite the center-right and co-opt the far-right National Front before regional elections in March. He has tried to play down the religious element in the debate, but he has also urged Muslims to show “humble discretion” and avoid “ostentation and provocation”; a junior minister, Nadine Morano, said young Muslims should dress better, find jobs and stop using slang and wearing baseball caps backward.