The worries about extremism are real. The Belgian far-right party, Vlaams Belang, took 20.5 percent of the vote in city elections last Sunday, five percentage points higher than in 2000. In Antwerp, its base, though, its performance improved barely, suggesting to some experts that its power might be peaking.

In Austria this month, right-wing parties also polled well, on a campaign promise that had rarely been made openly: that Austria should start to deport its immigrants. Vlaams Belang, too, has suggested “repatriation” for immigrants who do not made greater efforts to integrate.

The idea is unthinkable to mainstream leaders, but many Muslims still fear that the day — or at least a debate on the topic — may be a terror attack away.

“I think the time will come,” said Amir Shafe, 34, a Pakistani who earns a good living selling clothes at a market in Antwerp. He deplores terrorism and said he himself did not sense hostility in Belgium. But he said, “We are now thinking of going back to our country, before that time comes.”

Many experts note that there is a deep and troubled history between Islam and Europe, with the Crusaders and the Ottoman Empire jostling each other for centuries and bloodily defining the boundaries of Christianity and Islam. A sense of guilt over Europe’s colonial past and then World War II, when intolerance exploded into mass murder, allowed a large migration to occur without any uncomfortable debates over the real differences between migrant and host.

Then the terror attacks of Sept. 11, 2001, jolted Europe into new awareness and worry.

The subsequent bombings in Madrid and London, and the murder of the Dutch filmmaker Theo van Gogh by a Dutch-born Moroccan stand as examples of the extreme. But many Europeans — even those who generally support immigration — have begun talking more bluntly about cultural differences, specifically about Muslims’ deep religious beliefs and social values, which are far more conservative than those of most Europeans on issues like women’s rights and homosexuality.

“A lot of people, progressive ones — we are not talking about nationalists or the extreme right — are saying, ‘Now we have this religion, it plays a role and it challenges our assumptions about what we learned in the 60’s and 70’s,’ ” said Joost Lagendik, a Dutch member of the European Parliament for the Green Left Party, who is active on Muslim issues.