LONDON, Jan 7 — The sophisticated, military-style strike Wednesday on a French newspaper known for satirizing Islam staggered a continent already seething with anti-immigrant sentiments in some quarters, feeding far-right nationalist parties like France’s National Front.
“This is a dangerous moment for European societies,” said Peter Neumann, director of the International Center for the Study of Radicalization at King’s College London. “With increasing radicalization among supporters of jihadist organizations and the white working class increasingly feeling disenfranchised and uncoupled from elites, things are coming to a head.”
Olivier Roy, a French scholar of Islam and radicalism, called the Paris assault — the most deadly terrorist attack on French soil since the Algerian war ended in the early 1960s — “a quantitative and therefore qualitative turning point,” noting the target and the number of victims. “This was a maximum-impact attack,” he said. “They did this to shock the public, and in that sense they succeeded.”
Anti-immigrant attitudes have been on the rise in recent years in Europe, propelled in part by a moribund economy and high unemployment, as well as increasing immigration and more porous borders. The growing resentments have lifted the fortunes of established parties like the U.K. Independence Party in Britain and the National Front, as well as lesser-known groups like Patriotic Europeans Against Islamization of the West, which assembled 18,000 marchers in Dresden, Germany, on Monday.
In Sweden, where there have been three recent attacks on mosques, the anti-immigrant, anti-Islamist Sweden Democrats Party has been getting about 15 percent support in recent public opinion polls.
Paris was traumatized by the attack, with widespread fears of another. “We feel less and less safe,” said Didier Cantat, 34, standing outside the police barriers at the scene. “If it happened today, it will happen again, maybe even worse.”
Mr. Cantat spoke for many when he said the attacks could fuel greater anti-immigrant sentiment. “We are told Islam is for God, for peace,” he said. “But when you see this other Islam, with the jihadists, I don’t see peace, I see hatred. So people can’t tell which is the real Islam.”
The newspaper, Charlie Hebdo, in its raucous, vulgar and sometimes commercially driven effort to offend every Islamic piety, including the figure of the Prophet Muhammad, became a symbol of an aggressive French secularism that saw its truest enemy in the rise of conservative Islam in France, which is estimated to have the largest Muslim population in Europe.
The mood among Parisians near the scene of the attack Wednesday on the newspaper Charlie Hebdo was apprehensive and angry. “There’s no respect for human life,” said Annette Gerhard.
On Wednesday, Islamic radicals struck back. “This secular atheism is an act of war in this context,” said Andrew Hussey, a Paris-based professor of postcolonial studies. Professor Hussey is the author of “The French Intifada,” which describes the tangled relations between France and its Muslims, still marked by colonialism and the Algerian war.
“Politically, the official left in France has been in denial of the conflict between France and the Arab world,” Professor Hussey said. “But the French in general sense it.”
The attack left some Muslims fearing a backlash. “Some people when they think terrorism, think Muslims,” said Arnaud N’Goma, 26, as he took a cigarette break outside the bank where he works.
Samir Elatrassi, 27, concurred, saying that “Islamophobia is going to increase more and more.”
“When some people see these kinds of terrorists, they conflate them with other Muslims,” he said. “And it’s the extreme right that’s going to benefit from this.”
In reaction to the deadly attack on the French satirical newspaper Charlie Hebdo, President Obama said the United States would provide France with “every bit of assistance” in fighting terrorism.
The German interior minister, Thomas de Mazière, told reporters on Wednesday: “The situation is serious. There is reason for worry, and for precautions, but not for panic.”
With each terrorist attack, however, the acceptability of anti-immigrant policies seems to reach deeper into the mainstream. In Britain, for example, which also has a large Muslim population, the U.K. Independence Party has called for a British exit from the European Union and sharp controls on immigration, emphasizing what it sees as dangers to British values and identity. The mainstream parties have competed in promising more controls on immigration, too.
“Large parts of the European public are latently anti-Muslim, and increasing mobilization of these forces is now reaching into the center of society,” Mr. Neumann said. “If we see more of these incidents, and I think we will, we will see a further polarization of these European societies in the years to come.”
Those who will suffer the most from such a backlash, he said, are the Muslim populations of Europe, “the ordinary normal Muslims who are trying to live their lives in Europe.”
Nowhere in Europe are the tensions greater than in constitutionally secular France, with as many as six million Muslims, a painful colonial history in Algeria, Syria and North Africa, and a militarily bold foreign policy. That history has been aggravated by a period of governmental and economic weakness, when France seems incapable of serious structural, social and economic reform.
Several videos showing the gunmen outside the office of Charlie Hebdo, a satirical newspaper, have surfaced online. The footage includes scenes of graphic violence.
The mood of failure and paralysis is widespread in France. The Charlie Hebdo attack came on the publication day of a contentious new novel, “Submission,” by Michel Houellebecq, which describes the victory of Islam in France and the gradual collaboration of the society with its new rulers from within. Mr. Houellebecq, like the well-known caricaturists and editors who were killed at Charlie Hebdo, has been a symbol of French artistic liberty and license, and his publishers, Flammarion, were reported to be concerned that he and they could be another target.
But the atmosphere has been heightened by the rise of the National Front and its leader, Marine Le Pen, who runs ahead of the Socialist Party in the polls, campaigning on the threat Islam poses to French values and nationhood.
There was much recent attention to another best-selling book by a conservative social critic, Éric Zemmour, called “The French Suicide,” attacking the left and the state for being powerless to defend France against Americanization, globalization, immigration and, of course, Islam. Another new novel, by another well-known French writer, Jean Rolin, called “The Events,” envisions a broken France policed by a United Nations peacekeeping force after a civil war.
“This attack is double honey for the National Front,” said Camille Grand, director of the French Foundation for Strategic Research. “Le Pen says everywhere that Islam is a massive threat, and that France should not support attacks in Iraq and instead defend the homeland and not create threats by going abroad, so they can naturally take advantage of it.”
The military-style attack on Wednesday creates major security questions for France, said a senior French official who spoke on the condition of anonymity because he was not authorized to speak publicly on such a delicate matter. “We knew this would happen,” he said. “But we didn’t know how efficient it would be.”
Speaking in both English and French, Secretary of State John Kerry expressed solidarity with France against an attack on the French satirical newspaper Charlie Hebdo that killed 12 people.
After a series of three apparently lone-wolf attacks on crowds around Christmas in France, and other attacks in Ottawa and in Sydney, Australia, there was speculation that this attack might also be a response to the September call of a spokesman of the Islamic State, Abu Muhammad al-Adnani, for supporters to strike at domestic targets of the countries attacking the Islamic State.
Mr. Grand noted that at least 2,000 young French citizens have traveled to fight with the militants in Iraq and Syria. “So how do we manage our Muslim population?” he asked. “This kind of attack is very difficult to detect or prevent,” he said, adding that the state must not overreact, which is what the radicals want.
Still, he said, even given that the number of radical Muslims is a tiny minority in France, “there are definitely more than 50 crazy guys,” so it will be important to know whether the attackers had been to Syria or “wanted to go and did this instead.”
François Heisbourg, a defense analyst and special adviser to the Foundation for Strategic Research, in Paris, said that the professional military acumen of the attack reminded him of the commandos who invaded Mumbai, India, in July 2011. “This is much closer to a military operation than anything we’ve experienced in France, and that may limit the political impact,” he said.
“Between this attack and whatever real societal problems we have in France, there is a great gap,” Mr. Heisbourg said. “These were not corner-shop guys from the suburbs.”
The mood in Paris, near the scene of the attack, was both apprehensive and angry. Ilhem Bonik, 38, said that she had lived in Paris for 14 years and had never been so afraid. “I am Arab, Tunisian, Muslim, and I support the families, the journalists and all the people involved,” she said. “This is against Islam.”
When journalists are killed for expressing their views, it is one step away from burning books, said Annette Gerhard, 60. “It’s like Kristallnacht,” Ms. Gerhard said, noting that her family had died in Nazi deportations. “There’s no respect for human life.”
Rachel Donadio and Aurelien Breeden contributed reporting from Paris, and Alison Smale from Berlin.