http://www.skoazell-vreizh.org/galleg/renforcement_legislation.htmthis link gives 358 only in France
most are Basque (159) the others are Islamists (94) or Corsicans (76)
This is only the last years. If you go back to the early 90ies there were a lot of Islamists arrested and tried round 94-95 for the Paris bombings.
But if you count only with Islamists :
I heard the figure of 150 only for France for the last 5 years the other day on TV, this by the French anti-terrorist judge : 400 arrested, 150 convicted and jailed. All these for plotting. The trials are open and in most cases there are no doubts : explosives founds, weapons, chemicals etc...
I don't count with the 94-95 period where several (about 10) were shot while resisting arrests.
In Spain several have been convicted for participation (logistics) in the Madrid bombings and a bunch committed suicided by explosive during the arrest.
Some are convicted for the London bombings
There are cases in Holland, Germany, Italy too. The German cases involve several arrests related to 9/11.
Terrorism is nothing new in Europe, remember the IRA, The Bader-Meinhof league, the regional terrorism like Corsica etcc... The only difference is that the "Al Quaeda terrorism" tends to be on a spectacular scale.
A network has been found in my hometown (not mentioned here) this year : about 5 Islamists with explosives, detonators, AK-47, chemicals. This make hardly news...
That's why most Europeans consider the situation in the US hysterical, but the standard comment is "you have to understand them, they had 9/11"....