A group of 11 people took off their clothes, killed a sheep and chained themselves together by the main gate of the former Nazi death camp of Auschwitz, before being detained by police, officials have said.
A spokesman for the Auschwitz memorial, Bartosz Bartyzel, said their motives were not clear as they briefly stood and then sat down by the notorious Arbeit Macht Frei ("Work sets you free") gate in the southern Polish city of Oswiecim.
Auschwitz inmates passed through the gate when they were taken into the camp and when led by armed Nazi guards to slave labour sites.
Mr Bartyzel said museum guards ordered the group to dress and called police. Visits to the site were temporarily suspended.
A statement from the Auschwitz memorial's authorities said that "using the symbols of Auschwitz for any manifestations or happenings is reprehensible and outrageous" and "hurts the memory of all victims" of Auschwitz.
A spokeswoman for the local police, Malgorzata Jurecka, said 11 young people - Poles, Belarusians and a German - were detained for questioning on allegations they insulted a memorial site. If convicted, they could face stiff fines.
During the Second World War, the Nazis killed 1.1 million people at Auschwitz, mostly Jews, but also Poles, Roma and Russian prisoners of war among others.