Critics have said the museum housing the Ara Pacis altar is out of character
Vandals have splashed red and green paint onto the back wall of the controversial modern museum, the Ara Pacis, in Rome.
They also left a toilet outside and several rolls of toilet paper.
The museum, designed by the American architect Richard Meier, was opened in 2006 to house a 2,000-year-old carved marble altar of the Emperor Augustus.
Many criticised the building, in Rome's historic centre, as too modern, too large, and out of character.
The New York artist Julian Schnabel once famously described the museum as an "air conditioning unit".
And last year the new right-wing mayor of Rome Gianni Alemanno announced he wanted to hold a referendum on moving the museum to the city's outskirts.
Heat and pollution
The "Altar to Peace" commemorates the conquests of tribes in Gaul and Spain by the Emperor Augustus.
Pieces of it were rediscovered from the 16th Century onwards, and it was first re-housed in a pavilion opened by Mussolini in 1938.
During the 1980s the altar began to deteriorate in the heat and pollution of Rome and plans were laid for a new museum.
Although many criticised the exterior of the Ara Pacis, on the banks of the River Tiber, the interior design gained praise for its use of glass and natural light. It has also proved popular with tourists.
Workers have already started work to remove the paint from the back wall of the museum.