I saw a number mentioned in one newspaper article, and so went looking for more. I will add to this article as I discover more data.
Estimating the percentages of Muslims in prisons across Europe is not simple. In some cases, such as Sweden, collecting such data is illegal. In many cases, the numbers are only estimates.
I linked to my sources, but note that not all sources have the same reliability level. EUMAP - EU Research program, NEWS - mentioned in the news.
Belgium - 16% (12.8% Moroccans, 3.1% Turks) (EUMAP), 35% (NEWS)
Denmark - 20% (NEWS)
England+Wales - 11% (NEWS)
France - 50%-70% (NEWS, NEWS, NEWS)
Germany - 18% foreigners, lowest estimate for Muslims - 5% (EUMAP)
Italy - 40% foreigners, 8% Moroccans (NEWS), ~13% Muslims (NEWS)
The Netherlands: 20%, youth prisons - 26% (EUMAP)
Norway: Over 30% (NEWS)
Spain - 70% (NEWS)
Switzerland - 57.8% in Champ-Dollon prison (Champ-Dollon PDF)
Updated: August 6, 2009