THE children playing in the noisy streets of the Fosse aux Chenes neighborhood of Roubaix, an old textile city in northern France, are vivid testimony that immigration in Europe is no recent phenomenon.A bobbing mix of Italian, Portuguese, Polish, Turkish, and North African faces includes the grandchildren and great-grandchildren of immigrants who felt the pull of a prosperous Europe decades ago. Yet familiar as the issue may be, immigration has gained unusual attention in recent months, rising to the top of Europe's agenda. For reasons that are simultaneously economic, political, and cultural, polls show West Europeans place immigration among the major problems their countries now face. "It makes me laugh to hear people, including the politicians, talk about immigration like it is something very new," says 23-year-old Boudes Messaoud, as he watches the children of Fosse aux Chenes at play. "What has changed is the kind of people coming here, and maybe that's part of the problem," says Mr. Messaoud, himself the son of an Algerian immigrant. "But what is certain is that it's a problem that is going to grow all over Europe, because here people can work and live, and in many other places they cannot." The numbers of immigrants and immigrant neighborhoods like Fosse aux Chenes are growing across Western Europe. Legal immigration totals about 400,000 annually, not including the 3 million to 4 million illegal immigrants that call Western Europe home. As the numbers have grown, so have incidents thrusting immigration onto the front pages. In France during the past year, showdowns between police and North African youths left four people dead and sparked urban rioting. That has led the government to take emergency measures - security, youth employment, and social programs - to head off a "hot" summer. New French Prime Minister Edith Cresson sparked controversy last month when she suggested that planes be chartered to return illegal immigrants to their home countries. Brussels was also shocked by three days of rioting in April that pitted mostly North African youth against police. In Germany, and most recently in former East Germany, neo-Nazi youths stage regular demonstrations calling for the expulsion of "undesirable foreigners" and creation of a "pure" Germany. And Italy, which took in more than 24,000 Albanian refugees in March alone, slammed the door shut this summer. Reflecting growing xenophobia and fears about housing and job shortages, the government in June declared that Albanians will now be considered illegal aliens rather than political refugees. Still, it is not so much the numbers as it is the circumstances and makeup of Europe's immigration that make it an explosive issue. With the opening of Eastern Europe, Western Europe finds itself confronted with immigration on two fronts - one in the east, another in the south. * Hundreds of thousands of immigrants are expected to pour in from the east - particularly the Soviet Union, once its borders are completely open. * Political violence and continuing economic decline in much of Africa and parts of Asia push thousands of people annually into Europe. * Today two-thirds of immigrants coming to Europe are Muslim. A large majority are readily identified as such, either by physical traits or cultural habits. This increased flow of migrants has happened in a period of economic instability in which many West European countries face stubbornly high unemployment. It also comes just as the 12 members of the European Community (EC) are discussing ways to tighten political union - a move that many fear will weaken national sovereignty and identity. The continuing arrival of waves of immigrants - most from non-Western cultures - only heightens worries that Europe is economically and culturally under siege. As a result, there are growing calls across Western Europe for protection of the continent's identity and standard of living. Yet even if this were possible, experts insist it will remain difficult to agree on Europe-wide policies for addressing immigration. "It is difficult to compare immigration in Italy, which is something very new, with immigration in Germany, where Turks have been going to work for a long time, and where now reunification has changed the picture," says Jean-Pierre Garson, an immigration expert at the Organization for Economic Cooperation and Development in Paris. And even as cries mount to curb immigration, there are signs that an aging Europe needs to rejuvenate its work force. In the case of France, a INSEE report released last month finds that a population aging faster than previously thought means that the country will need more than 140,000 immigrants a year to fill out the work force in the first decade of the 21st century, and up to 180,000 a year by the third decade. "Immigration is a growing source of concern for many, because it is increasingly made up of people from the third world, from very different cultures," says Yves Gazzo, an immigration specialist for the EC and professor at the Political Studies Institute in Paris. Mr. Gazzo says the irony is that Europe's two-front immigration will lead to a de facto partitioning of immigrant destinations, with people from the East generally going north and those from the third world going to southern Europe and Britain. "Germany will become whiter and whiter by accepting more migrants from Eastern Europe," he says, "with France, to some extent southern Europe, and Britain becoming darker and darker." Europe could come to a point of conflict over who is accepting whom, Gazzo says, if the trend is borne out.