The 38-year-old man decided to go commando in front of a baffled crowd of spectators at Guggenheim’s Fish Room last Wednesday, online daily Qué reported.
The spontaneous stripper from the town of Beasain (Gipuzkcoa) removed first his trousers and then his underwear while being filmed by two other men who were accompanying him.
As soon as he'd bared all, the man broke into song, chanting the popular workers' movement song La Internacional, supposedly in honour of May Day celebrations.
Guggenheim staff were quick to alert the museum’s security personnel, who called local police to arrest the man.
The man was charged with public disorder, for which he will only have to pay a fine.