The Charlie Hebdo attack in Paris and growing support for a weekly anti-Islamization march in Dresden, Germany, have raised fears of an impending backlash against Muslims in Europe, many of whom already feel unfairly discriminated against.

On Thursday, German Chancellor Angela Merkel and French President François Hollande separately reaffirmed their support for Muslims in an attempt to counter the virulent rhetoric from anti-immigration groups.

Even so, police in Belgium, France, Germany, and Ireland arrested dozens of suspects Thursday and Friday amid fears that European Muslims returning from Middle East battle zones represented a terror threat in their home countries.

As views toward Muslims threaten to worsen across the continent, here are some basic facts to keep in mind:

How many Muslims live in Europe?

More than 20 million Muslims live in the 28 countries that form the European Union. Their share of the population across Europe grew from 4 percent in 1990 to 6 percent in 2010, according to the Pew Research Center. In 2030, Muslims are projected to make up 8 percent of the continent’s population.

About 4.7 million Muslims live in Germany and a similar number in France, the most of any countries in the EU. At 7.5 percent, France has the most Muslims as a proportion of its population in all of Western Europe. That figure is expected to increase to more than 10 percent by 2030. Muslims make up 5.8 percent of the population in Germany, 4.8 percent in Britain, and 3.7 percent in Italy. By comparison, Muslims in the United States make up 1 percent of the population.

How many Muslims do Europeans think live there?

Way more than actually do. A 2014 poll from the social research institute Ipsos Mori found that Europeans greatly overestimate the share of Muslims in the population. The average respondent from many European countries overestimated by a factor of three or more. For example, French respondents guessed 31 percent rather than 7.5 percent, and Germans guessed 19 percent rather than 5.8 percent.

As for the US? On average, American respondents thought 15 percent of their compatriots were Muslim.

Where do Muslim immigrants in Europe come from?

The EU is home to about 13 million Muslim immigrants who predominantly come from the Middle East, North Africa, Eastern Europe, and Turkey. France’s former colonies in North Africa – Algeria, Morocco, and Tunisia – are its largest source of foreign-born Muslims. The population in Germany is mostly made up of Turkish immigrants.

In recent years, an increasing number of refugees and asylum seekers from war-torn countries such as Syria and Iraq have boosted Europe’s foreign-born Muslim population. An estimated 200,000 people sought asylum in Germany in 2014, some 58 percent more than did so the previous year. The recent spike has led Germany to receive more asylum claims than any other industrialized country. It overtook the US for first place in 2013.

What do Europeans think of Islam?

Anti-Muslim sentiments are on the rise across Europe. A growing fear of Islam and pushback against new immigrants has led to the formation of extreme right-wing movements, many of which stand to gain support in the aftermath of the Paris attacks.

Such attitudes have been simmering across the continent for years. A 2013 survey conducted by the non-profit Bertelsmann Foundation in Germany showed that many Europeans view Islam as incompatible with the Western world, including two-thirds of respondents in Spain and just over half of respondents in France. A more recent survey found that 57 percent of non-Muslims in Germany perceive Islam as a threat, up from 53 percent in 2012. Forty percent said they didn’t feel at home in Germany because of the country’s purported Islamization.

“Muslims now consider Germany home,” Yasemin El-Menouar, who researches Islam at the Bertelsmann Foundation, said in a statement. “However, they find themselves confronted with a negative image that apparently prevails because of a minority of radical Islamists.”

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Do Muslims carry out a disproportionate number of extremist attacks?

No. Ethno-nationalist and separatist groups – think independence movements in Northern Ireland and the Basque region of Spain – carried out the vast majority of the 152 terror attacks in Europe in 2013, according to Europol, the EU’s law enforcement agency. A report released last year said only two attacks in 2013 were “religiously motivated.” And it's worth noting that one of the worst acts of terrorism to have occurred in Europe in recent history – the 2011 attack in Norway that left 77 people dead – was committed by a Christian extremist.

However, Europol did report a significant increase in arrests for religiously inspired terrorism, from 159 in 2012 to 216 in 2013, with many having ties to Islamic extremism. Many suspects were arrested for allegedly recruiting fighters for Islamic militant groups in Syria or traveling there to join them.