As the three young North African women talked about their Muslim faith at a cafe here one recent evening, they could not help noticing how patrons at the next table were reacting.
One French man leaned so far back in his chair to hear the animated discussion that he almost joined the group. Suspicion and disapproval darkened his look.
Nadia Mirad, a psychology student who works at a children's activity center, knows that look. Last year, she recalled, when she asked for a day off to celebrate the end of the annual Ramadan fast, her boss exploded.
"She said I was being unprofessional," Ms. Mirad explained, sipping a Coke. "She said the world didn't stop turning just for a Muslim holiday. I'm French, but I felt I was not a full French citizen at that moment. I really did not feel at home."
Her two student friends, both of them also born and raised in France, nodded in sympathy. "We feel as French as France will let us feel," said Bouthaïna Gargouri. "But it's true, I can't live my religion fully here."
None of them, for example, wears a head scarf, though they all say they would like to do so one day. Making such a visible show of their religion, however, would make it almost impossible for them to get a job, they agreed.
"I can't afford to put up barriers to what I want to be," said Leïla Bousteïla, who hopes to become an interpreter for deaf mutes.
Religion's place in public life has shot to the top of the agenda in France, and in the rest of Europe, for one reason: Islam, and the growing millions of people on the Continent who practice it.
Shocked by the discovery of Islamic terrorist networks on their soil, Europeans have suddenly woken up to the existence of an often marginalized Muslim minority that takes religion more seriously than they do.
Today, the relationship between native Europeans and their Muslim neighbors is fraught with tension. Mistrust on both sides threatens to explode into violence. Late last year, arsonists destroyed two mosques and a Muslim school in the Netherlands after an Islamic radical there was arrested for murdering filmmaker Theo van Gogh, who had criticized Muslim treatment of women.
Particularly unnerving are the violent messages spread by a number of radical Muslim preachers. "I believe the whole of Britain has become Dar ul Harb [land of war]," Syrian-born cleric Omar Bakri Mohammed told followers in a webcast on "PalTalk" last month. "The jihad is halal [acceptable] for the Muslims wherever they are." [Editor's note: The original version of this article mistranslated Dar ul Harb.]
"Active Christians in mainstream churches across the Continent are worried by the rise in fundamentalist nationalism," says Jorgen Nielsen, a professor of Islamic studies at Birmingham University in England.
"Secularists tend to be more worried not just about Islam but the return of religion to the public space," he adds.
Europe's Muslim population has tripled in the past 30 years, fueled by immigration from North Africa, Turkey, Pakistan, and Bangladesh. This rapid growth "questions our ... ability to integrate" them, warns Patrick Weil, a French sociologist.
"This is the first time for a long time ... that we have had to show that we can adapt and accept religious diversity," he adds. "That is a challenge."
At the same time, acknowledges Tariq Ramadan, one of the foremost Islamic thinkers in Europe, Muslims must change their thinking on many customs that alienate Europeans, such as their attitudes about women. "From Arab Islam, or African Islam, we have to come to European Islam," he argues.
Arguments over how to integrate Muslims into modern European life, and how much Islam Europe can accept without betraying its values, have been tainted by the link to terror. Governments have reacted by tightening controls on Muslim preachers, many of whom do not speak the language of their adopted country. Britain has introduced civics tests for imams. French authorities are planning to set up a school that would also send preachers in training to secular universities. And in Denmark, the right-wing People's Party, a government coalition member, urges a ban on all foreign imams.
Such moves have won support even in some Muslim quarters. "It is not xenophobic for Europeans to be genuinely worried about the radicalization of Islam," says Tim Winter, a British Muslim convert who teaches at Cambridge University and preaches at a mosque. "But it is not acceptable to say that Islam cannot adapt to European life."
Being religious at all, however, is unusual in European life. Though Muslims make up only 3 percent of the British population, more people attend Friday prayers than go to Sunday church, a recent survey found.
That scares many Europeans who fear that Europe could soon lose its Christian identity. The prospect of Turkey joining the European Union (EU) in 10 years' time, which would add an expected 83 million Muslims, deepens their fear.
"Europe is becoming Islamicized," warned Fritz Bolkestein a few weeks before he left his job as the EU's competition commissioner last December, noting that the two biggest cities in his native Netherlands, Amsterdam and Rotterdam, will be minority European within a few years.
That sounds like scaremongering to some Islamic leaders, who note that less than 5 percent of Europe's population is Muslim. To others, it sounds like a call to abandon their faith.
"Many European politicians, as well as average people, are prone to thinking that the only safe Muslims are those who neither practice their religion nor manifest their Muslim identity," wrote Mr. Ramadan in his book, "To Be a European Muslim."
Ramadan is the leading proponent of "European Islam," a school of thought intended to meet the needs of descendants of immigrants who have few ties to their ancestral cultures.
Last spring, Time magazine named him one of the 21st century's most influential people. But last summer, the US Department of Homeland Security controversially revoked his visa days before he was to begin teaching at the University of Notre Dame, in Indiana. A department official said Ramadan had been barred in accordance with a provision of the Patriot Act.
Ramadan insists that many of the habits Muslims display and that Europeans revile are not Islamic per se, but rather cultural traits specific to the Middle East, Africa, or Asia. "Muslims living in Europe have an opportunity to reread our [religious] sources," he says.
"We are going through a reassessment," he adds, "and the most important subject is women. Our experience in Europe has made it clear we must speak about equality."
"Europeanizing" Islam, says Professor Nielsen, whose home town, Birmingham, is knows as the "Muslim capital of Britain," "requires changes in relations between the sexes, in relations between parents and children, significant changes in attitudes to people of other religions, and in attitudes toward the state."
That is happening, Nielsen says. A few Muslims are assimilating completely with secular European culture, "but the majority are sticking to their religion but divorcing it from the cultural tradition and redressing it in a new culture."
At the same time, a small minority has turned toward a hard-line version of their religion, and a handful have taken up jihad, or holy war against the West. Police in several European countries have arrested hundreds of young Muslim men in connection with alleged terrorist plots since 9/11.
In Britain, Scotland Yard is investigating Mr. Bakri Mohammed after reporters heard him proclaiming that "death will be inevitable ... if people reject the call of mighty Allah" at a secret rally in London in January.
"There is a struggle for the soul of Islam," says Dr. Winter, also known as Sheikh Abdul Hakim Murad. Even as young European Muslims seek new ways of living their religion, "Gulf embassies ... spend tens of millions of pounds to ensure that the most fundamentalist form of Islam prevails in schools and bookshops," he laments. "Liberal Islam - economically, culturally, and socially - is crying in the wilderness."
The stronger fundamentalist Islam grows, the harder it will be for most Muslims to integrate, Ramadan says. "It is important for us as Muslims to be unambiguous that we respect the law and the secular framework," he insists.
On the other hand, he adds, Europeans "must start considering Islam as a European religion, and stop building a European identity against Islam as something external."
That will not be easy, given the secular European tradition of keeping religion out of the public space for fear that it might undermine democracy, a tradition developed in the face of an often reactionary Roman Catholic Church. It will be harder in the case of an unfamiliar religion often preached in a foreign tongue.
But Islamic thinkers hope that they can persuade Europeans that Islam has something to offer. "We are accused of encouraging the return of religious people to the public sphere," says Ramadan. "The question is whether we are ... contributing to society with concerns about values and ethics."
"If Islam cannot sit comfortably within the liberal European mainstream," says Winter, "it will raise the question whether Europe ... can accept substantial differences" among its citizens.
Back in the Paris cafe, Ms. Gargouri and her friends say it would not take much to make them feel more comfortable as European Muslims. For a start, suggests Gargouri, "people must stop confusing Islam with Islamism and even with terrorism. Islam was here long before 9/11."
Ms. Bousteïla agrees. "It would help," she says, "if I did not have a label stuck on me wherever I show up."