Fifteen people were injured when right-wing extremists attacked a May Day gathering in the German city of Weimar on Friday, according to police.
Around 29 people were arrested after the incident.
The attackers also targeted Social Democrat politician Carsten Schneider as he was trying give a speech, grabbing the microphone and shouting extremist slogans.
On his Facebook page, Schneider said he was in shock adding that people should wake up and not leave the country to the mob.
May Day demos by trade unions passed off peacefully in other parts of Germany. Activists called for the minimum wage to be protected against attacks from MPs.