After Berlin's incident at the Christmas Market, the other day (somewhat personal, given that I knew the streets involved, although it was never Christmas time when I was there), I've now observed armed police (as well as the regular bunch, being much more visible than usual) deployed to stroll around both the city-centre and the out-of-town shopping centre in pairs, here in the UK.
The connection seems obvious. We don't (out here in the provinces, at least) often see these guys walking round our British streets, and I've never noticed them in the latter locale. These serious-looking guys don't get deployed on a whim.
I wouldn't go so far as to say there's a credible threat, but maybe a credible fear, enough to make those in charge think it's less worrying for people to see them than it is to not see them. (Or, uncharitably, a political decision to make people think the situation is such that this is what they should be thinking.)
I don't think I've seen so many police firearms in one day since the day I happened to be wandering through the back streets of London to avoid a particularly rowdy anti-capitalism march in central London and happened to pass by an obvious staging post for backup patrols of various types (riot, equine, firearms, paramedics... no firefighters though), waiting to see if they were needed. (And they almost certainly weren't, despite how the march later turned out on the news.)
I imagine that the DefCon level is similarly raised all over, this week, across Europe as a whole.