Vandals threw paint-filled balloons at a controversial museum designed by U.S. architect Richard Meier overnight, splashing the red and green colors of the Italian flag against the outside white wall, police said.

The vandals also left a porcelain toilet and two packs of toilet paper next to the Ara Pacis museum, which sits on a fascist-era piazza in central Rome, police said.

The white, block-like structure has attracted controversy since the project was assigned to Meier in 1998 by the center-left municipal administration in power at the time. Critics contend the modern building clashes with Rome's classical architecture.

Rome's right-wing mayor Gianni Alemanno, who last year threatened to move the building because he didn't like it, condemned what he called an "irresponsible and idiotic" act of vandalism.

"Hooliganism and vandalism won't decide the debate over interventions on architecture and monuments in the city," Alemanno said, adding that officials were currently in talks with Meier to discuss changes to the building to "improve its urbanistic impact."

The museum houses a carved sacrificial altar built in 9 B.C. to celebrate the peace that Rome's first emperor, Augustus, brought to his people. It was broken up and lost for hundreds of years until pieces began resurfacing in the 16th century.

Fascist dictator Benito Mussolini, who strove to connect fascism with the glories of the Roman Empire, ordered the altar fully excavated in the 1930s to commemorate the anniversary of Augustus' birth.

Mussolini inaugurated a museum housing the tomb in 1938, placing it just behind Augustus' mausoleum on a piazza that remains the only fascist-era construction in Rome's historic center. That museum was torn down after it fell into decay, and was replaced with Meier's building.

Police said Monday they have not determined a motive and that no one has claimed responsibility. By early Monday afternoon, public works officials were already repainting the green and red splashes of paint white again.

One of the onlookers at the site Monday was artist Graziano Cecchini, known for having tinted the waters of the Trevi Fountain red and tossing colored balls down the Spanish Steps. Cecchini, who is running for a right-wing party in European Parliament elections, said he loved the gesture, which he said was more art than politics.

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

An email seeking comment from Meier's New York studio wasn't immediately returned.

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Associated Press reporter Andrea Dessi contributed to this report.