Two atrocities in the space of the year, coming from opposite ends of the spectrum, are raising fears across Europe that a growing climate of ethnic and religious hostility is inspiring extremist violence - and creating the conditions for deadly clashes.
The attacks in France and Norway represent the most horrific extremes of two trends of intolerance troubling Europe: Strengthening far-right sentiment that has sometimes bled into the mainstream, and growing Islamic radicalisation in Europe's disadvantaged, immigrant-heavy neighbourhoods.
With Europe still stunned by last week's killings in Toulouse, France, a loosely knit group of xenophobic "defence leagues" plans to rally in Denmark on Saturday against what they call the growing Islamic presence in western Europe.
The rally was organised by one of the rising forces of Europe's far-right scene - the Danish Defence League. It's backed by the English Defence League, which gained prominence in Britain amid urban rioting last summer.
Similar groups from Russia, Finland, France, Germany, Norway, Poland, Romania, and Sweden are expected.
Danish intelligence services expect up to 700 of these strident, anti-Muslim "counter-Jihadists". A counter-demonstration is anticipated to draw several thousand people. Police vow to keep the two groups apart. But the clashing views on display show Europe's heightened polarization.
"These terrorist events are creating sparks, and a small spark can set off a huge fire," said Magnus Ranstorp, research director of the Centre for Asymmetric Threat Studies in Sweden. "It can set off huge social polarisation, and this is what the terrorists want to achieve. Now there is an increased rightwing climate - the counter-jihad movement - feeding off these Islamophobic forces."
The mood is volatile, Ranstrop said, made more so by the methods of the killers - citing how in France, Mohamed Merah shot video of his attacks that was mailed to the Al-Jazeera television network.
"You have the counter-jihad movement, and on the other side you have an old al-Qaeda structure giving out directives for people to carry out their own personal jihads by solo terrorist activity. The manner in which you carry out these attacks matters: Recording them, sending films to Al-Jazeera, shooting people execution style, all to create polarisation and revulsion, to create an overreaction."
For decades, western Europe has been the envy of the world with its high standard of living and tolerant social climate. Today, Europe is gripped by a profound economic crisis and festering conflict over immigration, religion and cultural identity.
Tensions over immigration from northern Africa and other countries with large Islamic populations have fuelled the rise of far-right movements across Europe. In France, the ultranationalist National Front is expected to make gains in upcoming presidential and legislative elections.
Xenophobic parties in Austria, Finland, Sweden and the Netherlands have all gained support in recent years.
As anti-immigration rhetoric grows more strident, ideas that were once considered on the fringes of political dialogue have entered the mainstream - with French President Nicolas Sarkozy often seeming to borrow from National Front rhetoric as he campaigns for re-election.
At the same time, anti-Western diatribes on the internet and sometimes in local mosques have played a role in radicalising some young Muslims in Europe, even as Muslim community leaders try to steer young people toward productive futures.
The long wars in Afghanistan and Iraq have also enflamed passions among Muslims.
Over the past year, Europe has suffered deadly manifestations of these tensions with the France attacks and the July massacre in Norway carried out by Anders Behring Breivik, who slaughtered 77 people with a bomb in Oslo followed by a shooting rampage at a Labour Party youth camp on an island retreat.
The two horrors have drawn inevitable comparisons.
Both killers had a terrifying sense of theatre. Breveik's island rampage, in which he picked off the trapped children one at a time, seemed straight out of a horror movie. The French police account of Merah's last stand, his Colt .45 blazing as he jumped from the balcony, seems inspired by an action film.
Initial reactions to the attacks point to the ambiguities swirling around Europe's immigration debate.
When Breveik's deadly bomb ripped through the Norwegian capital on July 22, many counterterrorism experts assumed Muslim radicals were behind the blast. Similarly, many initially suspected that the French killings might be the work of a far-right fanatic.
The bloodshed carried out by both extremes in the heated immigration debated raises the spectre of clashes between the two. There already have been street scuffles between the English Defence League and radical Islamists in Britain - although none has escalated into major violence.
Nicolas Lebourg, a historian who studies the far-right at Perpignan University in southern France, said that both Breivik and Merah were products of an increasingly polarised Europe.
"For people who are a little fragile, people who are a little sensitive ... we're overheating them by telling them that there's this cosmic war between good and evil," Lebourg said.