A GROUP calling itself "Hooligans Against Salafists" had dutifully registered to stage a legal demonstration against jihadist Islam in central Cologne on October 26th. But on Facebook and Twitter the call quickly spread. Ultimately some 4,800 people showed up, most of them male and looking for trouble, gathering between Cologne's train station and its famous cathedral. A right-wing band called Kategorie C started playing. (Category C is German police jargon for football fans looking for a fight, as opposed to Category A, who actually want to watch football, and Category B, who while not hell-bent on fisticuffs are open to a scuffle if one presents itself.) Neo-Nazis pranced about, giving the Hitler salute. They included "SS-Siggi", a blond giant who is a celebrity in fascist circles. Beer bottles started flying. Soon, a full-fledged street battle pitted violent hooligans against 1,300 cops, about 50 of whom ended up injured. The images, reminiscent of street fighting during the Weimar republic of the 1920s, shocked Germans. Some called for banning similar demonstrations in the future. But Germany has deeply internalised the lessons of its past, including the sanctity of freedom of association. Can and should authorities simply ban such gatherings if they suspect that violent types plan to mix in?

The question is far from academic, because the same groups who organised the Cologne riots, or individuals who could be linked to them, have already registered similar "demonstrations" for November in Berlin and Hamburg. Meanwhile, the internet is atwitter with hints that neo-Nazis plan to gather in front of Berlin's Reichstag on November 9th, the anniversary of the fall of the Berlin Wall. Nor is the problem limited to right-wing extremists: left-wingers and various embattled Muslim communities would also like a show of force.

All this means hand-wringing for Germany's political class. Decisions to approve demonstrations are made by the interior ministers of the 16 states. Germany must not let extremists make a joke out of its democracy, goes one argument. Germany must not overreact and limit freedom of speech, goes the rebuttal. In the background is a rising German anxiety that the struggles of the world, and in particular the conflicts in the Middle East, will also increasingly be waged on their own streets.