BERLIN- Germany's Social Democrats (SPD) evacuated their headquarters building in Berlin on Tuesday after a bomb threat the party said was linked to its Chairman Sigmar Gabriel's visit to a town where anti-foreigner violence has erupted.Yasmin Fahimi, the SPD's general secretary, said that the centre-left party that shares power with Chancellor Angela Merkel's conservatives received many threats with racist overtones after Gabriel visited the eastern town of Heidenau on Monday."Since Sigmar Gabriel's visit to Heidenau, the Willy-Brandt-Haus has received a flood of racist threats," Fahimi said in a statement, referrring to the headquarters. "We've got to assume that this is a political act. Let us be clear about this: the SPD won't back down from its stance."More than 30 police were injured in clashes in Heidenau, near Dresden, early on Saturday when a mob of several hundred people, many of them drunk, began pelting officers with bottles and fireworks. Some shouted "Heil Hitler".Gabriel had said in Heidenau that Germany would not "give a single millimetre to these far-right mobs" and added those in Heidenau attacking refugees "have nothing to do with Germany. This is not the Germany we want."