CRÉTEIL, France -- The spacious and elegant modern building, in the heart of this middle-class suburb of Paris, is known as "the mosque of the converts."
Every year about 150 Muslim conversion ceremonies are performed in the snow-white structure of the Sahaba mosque in Créteil, with its intricate mosaics and a stunning 81-foot minaret, built in 2008 and a symbol of Islam's growing presence in France. Among those who come here for Friday Prayer are numerous young former Roman Catholics, wearing the traditional Muslim prayer cap and long robe.
While the number of converts remains relatively small in France, yearly conversions to Islam have doubled in the past 25 years, experts say, presenting a growing challenge for France, where government and public attitudes toward Islam are awkward and sometimes hostile.
French antiterrorism officials have been warning for years that converts represent a critical element of the terrorist threat in Europe, because they have Western passports and do not stand out.
In October, the French police conducted a series of antiterrorism raids across France, resulting in the arrests of 12 people, including at least three French citizens who had recently converted to Islam. Converts "often need to overdo it if they want to be accepted" as Muslims, and so veer into extremism more frequently than others, said Didier Leschi, who was in charge of religious issues at the Interior Ministry under former President Nicolas Sarkozy.
There are persistent concerns that French prisons are fertile ground for conversions and for Islamic radicalism; observant Muslims are thought to make up a least a third of the inmate population, according to French news reports.
Many Muslims counter that they regularly face prejudice, and consider a 2010 law banning the full-face veil from public spaces and the growing concern with conversions as reflections of French intolerance.
Whatever the impact, there is little doubt that conversions are growing more commonplace. "The conversion phenomenon is significant and impressive, particularly since 2000," said Bernard Godard, who is in charge of religious issues at the Interior Ministry.
Of an estimated six million Muslims in France, about 100,000 are thought to be converts, compared with about 50,000 in 1986, according to Mr. Godard. Muslim associations say the number is as high as 200,000. But France, which has a population of about 65 million, defines itself as secular and has no official statistics broken down by race or creed.
For Mr. Godard, a former intelligence officer, it is the "nature" of conversions that has changed.
Conversions to marry have long been common enough in France, but a growing number of young people are now seen as converting to be better socially integrated in neighborhoods where Islam is dominant.
"In poor districts, it has become a reverse integration," said Gilles Kepel, an expert on Islam and the banlieues, the poor, predominantly Muslim neighborhoods that ring Paris and other major cities.
Many converts are men younger than 40, experts say, often born in France's former African colonies or overseas territories.
Charlie-Loup, 21, a student from nearby St.-Maur-des-Fossés, converted to Islam at 19, after a troubled adolescence and strained relations with his mother. He grew up Roman Catholic but had many Muslim friends at school. "Conversions have become a social phenomenon here," he said, asking that his surname not be used because he considered his conversion a private initiative and did not want to draw attention to himself. Some convert simply "out of curiosity," he said.
In some predominantly Muslim areas, even non-Muslims observe Ramadan, the Muslim holy month that requires fasting during the day, because they like "the group effect, the festive side of it," said Samir Amghar, a sociologist and an expert on radical Islam in Europe.
In many banlieues, Islam has come to represent not only a sort of social norm but also a refuge, an alternative to the ambient misery, researchers and converts say.
For Mr. Amghar, Islam provides more structure and discipline than other religions. It is a way to "refuse modernism," get back to a society with more family values and a clearer distinction between men and women. "Islam has a peaceful effect on the converts," Mr. Amghar said. "The world looks clearer after they've converted."
In Marseille, on the southern coast, "conversions have increased at an incredible pace in the last three years," said Abderrahmane Ghoul, the imam of the major mosque of Marseille and the president of the local branch of the French Council of the Muslim Faith. Mr. Ghoul signed about 130 conversion certificates in 2012.
Hassen Chalghoumi, the moderate imam of Drancy, another suburb near Paris, says he thinks conversions have also been propelled by France's official secularism, which he says breeds spiritual emptiness.
"Secularism has become antireligious," Mr. Chalghoumi said. "Therefore, it has created an opposite phenomenon. It has allowed people to discover Islam."
Many experts note the influence of celebrity converts, particularly soccer players. Nicolas Anelka, who played on the French national team and whose parents came from Martinique, changed his name to Abdul-Salam Bilal Anelka when he converted to Islam in 2004. Franck Ribéry, a popular player from northern France, converted to Islam in 2006 to marry a Muslim woman, Wahiba, and took the name Bilal Yusuf Mohammed.
But there is rising anxiety here about the influence of Islam, especially conservative Salafist Islam, particularly among those on the center-right. Islam is regularly at the center of heated debates about the nature and future of France and its culture, and politicians can win attention and support by criticizing the expansion of Muslim customs into the wider public sphere: for example, the rise of women-only sessions in public swimming pools or the increasing availability of halal food.
In 2009, a photograph from the magazine Paris Match showing Diam, a popular female rapper, wearing a hijab, or head covering, on a Paris street set off a flood of angry comments from officials and commentators. Fadela Amara, a former secretary of state for urban affairs and founder of a feminist group, Neither Whores Nor Submissives, said that the hijab sent out a "negative image of women" and described it as "a real danger for young women in poor districts."
But Diam's dismissed her critics, saying that having her hijab did not make her a radical Muslim, and that her conversion was a personal choice that had helped her with depression.
Recent arrests of radical Muslim converts have also increased concern among public officials and Muslim leaders, though radical Islam is by no means the norm among converts.
Rafaello Sillitti, the owner of the bookstore Averroès, which occupies a small space in the Créteil mosque, is convinced that converts like him can be the best advocates of Islam. He sells carpets equipped with compasses to help users orient themselves toward Mecca and a wide range of books written by Muslim scholars, with titles like "Be Master of Your Physical Desire" and "How to Use a Cellphone According to Islamic Law."
"We must get rid of an imaginary Islamic culture," Mr. Sillitti said, referring to the clichés and misapprehensions connected to Islam in France. "We must show that French culture and Islam can live together in peace."
Correction: February 4, 2013, Monday
This article has been revised to reflect the following correction: An earlier version of this article referred incorrectly to one celebrity convert, Nicolas Anelka. Mr. Anelka once played regularly for the French national soccer team, but has not done so since he set off an uproar during the 2010 World Cup by insulting the team coach.world
This article originally appeared in The New York Times.