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The problems began in July when news came of the planned school for 200 students not far from the city center.

“That’s when we decided we had to take action,” Mr. Schultz said. He and about 250 others staged weekly protests on the market square. They collected thousands of signatures and formed alliances with local lawmakers, church leaders and moderate Muslim imams.

A few weeks ago, the city banned further construction on the school, citing security concerns. The congregation is suing to have the decision reversed.

Invitation to Paradise says its membership comprises immigrants and native German converts to Islam.

“We don’t belong to al Qaeda, we’re not terrorists and we don’t oppress women,” Adnan Beslija, a 29-year-old Bosnian, said at Zam Zam, the group’s shop in Moenchengladbach, whose wares include caftans, veils and package pilgrimages to Mecca, Saudi Arabia.

Mr. Beslija, who has a long, brown beard and a dark prayer mark on his forehead, says his congregation has the right to practice its religion freely just like Christians, Buddhists or Jews.

German intelligence services, however, say they have an eye on Invitation to Paradise because it belongs to the Salafist movement, which often has been linked to terrorist plots and seeks to revive strict Muslim doctrine dating back to the era of the 7th-century Prophet Muhammad.

“Not every Salafist turns into a terrorist, but we know that future terrorists have almost always visited Salafist workshops and schools,” writes the intelligence service of the state of Lower Saxony, where Invitation to Paradise also is active.

“Their indoctrination has a radicalizing influence, and that’s one reason why these schools are dangerous,” it says in its 2009 report.

So far, Germany has not been attacked by Islamic terrorists, but several plots have been foiled, and last month the government raised its security level nationwide.

German converts and young immigrants are considered a particular security threat by intelligence services. Germany’s Federal Criminal Police office says it has “concrete evidence” that 70 have traveled to Pakistan’s lawless border region for terrorism training in recent years, and about a third have returned to Germany.

Mr. Schultz is a soft-spoken, bespectacled man who studied theology and law, has three children and lives in a book-filled 19th-century town house. “We are not some far-right fringe movement against foreigners,” he said. “We are moderate Germans: businessmen, academics, store owners.” His group also has Italian and Spanish members, he said.

What his group is doing may once have seemed out of place in Germany, but today it reflects a broader trend.

A catalyst for protest has been a recent book by a pillar of the establishment, former central bank board member Thilo Sarrazin, which uses blunt, often harsh language to portray Muslim immigrants as welfare cases weakening German society and making it “dumber.” Although condemned by Mrs. Merkel and other leading politicians, it is a best-seller that has struck a chord among many Germans who fear their language, culture and generosity are being abused.

Last month, Interior Minister Thomas de Maiziere attended a town hall meeting in Moenchengladbach organized by Mr. Schultz and said the government was checking whether Invitation to Paradise could be banned as unconstitutional.

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