OSLO — This is one of the most placid and pleasant capitals of Europe, but Oslo is a divided city. The west of the city is rich, safe and predominantly white; the east, poorer, less safe and populated by immigrants, many of them Muslim.

Norway has recently tightened its liberal immigration and asylum rules in the midst of a longstanding debate about assimilation and multiculturalism. Despite Norway’s oil wealth and low unemployment, there has been a growing concern over the increasing size of the Muslim population, especially after Sept. 11 and the Danish crisis over the publication in late 2005 of cartoons depicting Muhammad, which were published in Norway, too.

But the Muslim population is growing, and Islam is now the country’s second-largest religion. The impact of an increasing, and increasingly visible, Muslim population in a relatively monoethnic, liberal and egalitarian Norway has led to a surge in popularity for the anti-immigration Progress Party, now the second-largest party in Parliament. And it appears to have been one of the triggers to the massacre carried out here on Norway’s white elite. The suspect, Anders Behring Breivik, claims he was compelled to act by the failure of mainstream politicians — including those in the Progress Party — to stem the Islamic tide.

In many ways, such arguments seem absurdly inflated. Norway, with 4.9 million people, has some 550,000 immigrants, about 11 percent of the population, but 42 percent of them have Norwegian citizenship. Half of the immigrants are estimated to be white Europeans, especially Poles and Swedes, coming to get better wages in rich Norway.