The Kunstverein in Hamburg said the incident is currently being investigated by authorities as a hate crime

Exhibition wants to show the "making of" the nihilist philosopher's personality cult

The painting, Madame Soler, was previously owned by the prominent collector Paul von Mendelssohn-Bartholdy in the 1930s

Use of police forces against supporters of artist Sandro Sulaberidze have been criticised by country's top state officials

The paintings, from the workshop of the Flemish master Dieric Bouts, were transferred from the Museo Provincial de Pontevedra in Spain to Gołuchów Castle

Statement comes after Berlin state secretary for diversity and anti-discrimination spoke out in favour of restituting the artefacts to their countries of origin