In the Rhine Valley city of Mannheim, the glittering minaret of Germany's biggest mosque overshadows what was once the region's most vibrant church, testifying to Muslims' new confidence as Christian churches are closing down.

Years ago, 180 sisters of the Catholic order of the Sisters of the Divine Savior were the pulse of the city. Today, eight remain. Every weekend, roughly 150 Roman Catholics attend mass at the Liebfrauen Church, while up to 3,000 Muslims throng the Yavuz-Sultan-Selim mosque. Since the mosque was opened in 1995, Muslim shops and youth centers have become a magnet for the Muslim community.

Mannheim is not unique. Across Europe, the Continent's fastest-growing religion is establishing its public presence after decades in basements and courtyards, changing not only the architectural look of cities, but also their social fabrics.

Hailed by many as a sign of Muslim integration, the phenomenon is also feared as evidence of a parallel Islamic world threatening Europe's Christian culture.

"Muslims have come out ... and have become visible," says Claus Leggewie, a political scientist at Germany's University of Giessen who wrote a study on the evolution of the mosque landscape in Germany. "By building expensive, representative mosques, they're sending a message: we want to take part in the symbolic landscape of Germany. We are here and we'll stay here."

Major mosque projects from Cologne, Germany, to Amsterdam to Seville, Spain, have met with fierce opposition and fears that they will serve as breeding grounds for terrorists. Family members of two of the suspects in the Glasgow, Scotland, car bombings this month said the men had been radicalized by Tablighi Jamaat, an Islamic revivalist group with plans for an 18-acre complex near London's 2012 Olympic stadium that would house Europe's largest mosque.

A local debate in Wiesbaden

Such a structure is a far cry from the dark, cramped basement that hosted Halif Kuzpinar's Friday prayers for 33 years after he left his native Turkey to work on Frankfurt's roads. Then, the Muslim group he belongs to bought a vacant supermarket in a residential neighborhood of Wiesbaden, a city in central Germany.

"There are parking spots. Children can come. There are better facilities for the youths," says Mr. Kuzpinar. "We want to build something nice so that people can come and see what we're doing."

With a place of its own, Milli Gorus – an Islamic Turkish rights group watched by the German government – is looking for something it never had: public recognition in a country its members consider theirs.

But it also ignited vehement protest. "This was to be sold as a supermarket, not as a mosque," says Wolfgang Kopp, who owns an apartment across the street. Along with other neighbors, he succeeded in, at least temporarily, stopping the parcel's rezoning for religious purposes. "Sooner or later there will be problems," he says.

Under the German Constitution, all religious groups can have prayer facilities. While Muslims have had prayer rooms, says Klaus Endter, an ecumenical specialist for the Protestant Church in Hessen, "the question is ... whether a courtyard is the right place to exercise a religion."

Guest workers worship more openly

Since coming to Germany, Muslim migrant workers like Mr. Kuzpinar have held prayer meetings in dark nooks that reflected the precarious situation of a people often torn between their adopted and their home countries.

But the "guest workers" who helped drive the economic boom of postwar Germany stayed. They set up organizations to run prayer, youth, and senior activities. They moved up the economic ladder, increasing their financial contributions to the groups, and receiving funds from pan-European Muslim organizations supporting the Muslim diaspora.

And now, the third generation is building domed mosques with minarets. Only a handful existed 10 years ago, but today 159 mosques dot Germany today, with 184 under construction, according to the Central Institute for Islamic Archives in Söst.

Aachen, for instance – a German city of 257,000 on the Belgian border with a 9 percent Muslim population – just gave the green light to a domed mosque with a minaret. That's a sign, says Mayor Jürgen Linden, "that Muslims have become a part and parcel of society."

But many see the arrival of mosques as a threat, with fears and conflicts worsening since 9/11, argues Mr. Leggewie, the mosque specialist. Grass-roots initiatives have sprouted that try to thwart mosque projects.

"I have the responsibility to protect our society, our democratic principles, ... our values," says Regina Ebenich, who leads an antimosque effort in Wiesbaden. Why, for instance, she asks, can't Muslim girls take part in swimming lessons or attend class field trips?

"A mosque is never a religious place only," says Willi Schwend, head of the antimosque National Association of Citizens' Initiatives. "A mosque is a caldron of political agitation. The goal of Islam is to spread the principles of Islam into society, to change society, to bring about sharia [Islamic law]."

Mr. Schwend's call finds an echo in eastern Berlin, where plans for a mosque with a 40-foot-high minaret have enraged a 6,500-inhabitant neighborhood. Although Berlin has numerous mosques, this would be the first in the former communist part of the country. There, argues Leggewie, the absence of a tradition of immigration, combined with strong right-wing feelings, explain why fears of Islam run deep.

A conflict of cultures

But even in Cologne, in the western part of the country, plans for what would become Germany's biggest mosque – with two 170-foot minarets slated to accommodate 2,000 people – has ignited a conflict of cultures.

With more than 100,000 Muslims living in Cologne, Germany's fourth-largest city, many religious and political leaders have rallied around the mosque plan. But Ralph Giordano, a prominent writer and Holocaust survivor, rekindled fears of a radical Islam threatening German society. "The integration of Muslims has failed," Giordano told the media.

Endter says Germany's mainstream population can no longer afford to ignore that it lives in a country of immigrants.

"You can't say, on the one hand, "We invite you to work, come over,' and on the other hand say, "Yes, you can pray, but only in courtyards, basements, in the shadow of society,'" he says. "We are in a phase of upheaval. The Muslim communities want to integrate. They don't want to live in the shadow anymore."