Workers paint over the splatters on the back wall of the museum designed by U.S. architect Richard Meier. (Riccardo De Luca/Associated Press)The Ara Pacis museum, a controversial modern museum in central Rome, has been hit by vandals with paint-filled balloons.

Vandals left a porcelain toilet and two packs of toilet paper next to the museum in the overnight attack, which appeared to be a comment on the design.

The paint on the white, block-like structure was red and green — the colours of the Italian flag.

There have been objections to the modern outline of the building by Richard Meier since the U.S. architect was hired to design it.

Right-wing Mayor Gianni Alemanno last year threatened to move the building which was a project of his predecessor. His criticism is that the modern structure doesn't fit with Rome's classical architecture.

The mayor has begun negotiations with the architect in an attempt to change the design. And he condemned the vandals.

''Hooliganism and acts of vandalism will not influence the debate on architectural works in the city,'' he said. Workers began painting over the splatters on Monday.

A view of the Ara Pacis museum at its reopening in Rome on April 21, 2006. (Pier Paolo Cito/Associated Press)No one has claimed responsibility for the vandalism, but suspicion has fallen on Graziano Cecchini, the man who tipped red dye into the Trevi Fountain in 2007 and sent a half a million coloured balls down Rome's Spanish Steps in 2008.

Cecchini, a right-wing activist, welcomed the sentiment of the attack on the museum, but did not claim responsibility.

"I don't think it's an act of vandalism," he said at the site of the paint spatters. "Look, it's beautiful. I'd do one a day."

Police say they have not determined a motive, or made an arrest in the case.

Meier's museum replaces a neoclassical one, erected in 1938 by Fascist dictator Benito Mussolini, that fell into disrepair.

It houses a carved sacrificial altar dating from 9 B.C . called the Ara Pacis, because it was built in the time of peace presided over by Roman emperor Augustus.

With files from the Associated Press