Butchered piglets hang in tidy rows at the open-air market, and shoppers haggle over cheese and oysters in a scene hardly altered since the last Bourbon king was buried at the Gothic church on the corner.

But slip out of the market on a Friday, and a quarter-mile up the road you will find a very different France: Hundreds of Muslims squeezed hip to hip into an unheated canvas tent, bowing in sacred silence toward Mecca, the birthplace of Islam, which few of them have ever seen.

The worshipers at this makeshift mosque on the edge of Paris are men and women, dressed in the latest fashions and traditional robes, Arab, European and African. They are moderate, conservative and fundamentalist. They are first-, second- and third-generation immigrants. They are content and they are enraged. They are the future that Europe is straining to handle.

What is happening in Europe may provide a partial preview of what lies ahead for the United States and its fast-growing Muslim population.

For the first time in history, Muslims are building large and growing minorities across the secular Western world--nowhere more visibly than in Western Europe, where their numbers have more than doubled in the past two decades. The impact is unfolding from Amsterdam to Paris to Madrid, as Muslims struggle -- with words, votes and sometimes violence--to stake out their place in adopted societies.

Disproportionately young, poor and unemployed, they seek greater recognition and an Islam that fits their lives. Just as Egypt, Pakistan and Iran are witnessing the debate over the shape of Islam today, Europe is emerging as the battleground of tomorrow.

"The French are scared," said Tair Abdelkader, 38, a regular at the tented mosque whose light blue eyes and ebony beard are the legacy of a French mother and Algerian father. "In 10 years, the Muslim community will be stronger and stronger, and French political culture must accept that."

By midcentury, at least one in five Europeans will be Muslim. That change is unlike other waves of immigration because it poses a more essential challenge: defining a modern Judeo-Christian-Islamic civilization. The West must decide how its laws and values will shape and be shaped by Islam.

For Europe, as well as the United States, the question is not which civilization, Western or Islamic, will prevail, but which of Islam's many strands will dominate. Will it be compatible with Western values or will it reject them?

Center stage in that debate is France, home to the largest Islamic community on the continent, an estimated 5 million Muslims. Here the process of defining Euro-Islam is unfolding around questions as concrete as the right to wear head scarves and as abstract as the meaning of citizenship, secularism and extremism. In some cases, conservative Muslims have refused to visit co-ed swimming pools, study Darwinism or allow women to be examined by male doctors.

One young St.-Denis fundamentalist recently set off for Iraq and was captured fighting American troops in Fallujah. Stunned by stories like that, France is hoping to use the legal system to influence the direction of Islam within its borders.

The government has deported 84 people in the past six months on suspicion of advocating violence and drawn wide attention for banning head scarves and other religious symbols in public school. But even supporters of that tough approach concede that the measures can do little more than patch the widening cracks in Europe's image of itself.

"I'm not sure we'll go much further than gaining a few months or years" in the effort to limit Islam's imprint on France, said Herve Mariton, a member of the French Parliament who lobbied for the head scarf law. "That may be useful. But there is no way this is the ultimate answer to the challenge."

A new France

St.-Denis' narrow streets sweep outward from a soaring 12th Century basilica that is the final resting place for generations of French monarchs. But today their snowy stone statues stare down onto a city and nation in transformation.

The Muslim migration to Europe began in earnest after World War II, when North African workers arrived by the thousands to help rebuild the continent. A half-century later, no fewer than a third of St.-Denis' 90,000 residents are of Arab origin.

Arabic script on butcher shops and storefronts touts halal meat, handled to Islamic standards. Couscous restaurants are as plentiful as brasseries. Muslim settlement houses usher in new immigrants, and Muslim funeral homes bid farewell to old ones.

Across the country, French Muslims still live more or less where the first arrivals settled a half-century ago, in suburban apartment blocks erected in the 1950s for foreign workers. These suburbs, the banlieues, have become the byword for France's virtually segregated Muslim communities.

The complexes used to be integrated, with Polish, Italian and French workers living among North African arrivals, but over time the Europeans moved on--and the Arabs did not. It is a scene repeated across the suburbs of Paris.