City Hall is banning all those enjoying a Roman holiday this summer from snacking near the sights in Rome's historical center with fines up to $80.
Officials say they want to preserve artistic treasures and decorum in a city that has millions of visitors every year.
The ordinance also bans the homeless from setting up makeshift beds and cracks down on drunks, litterbugs and nighttime revelers loitering in central areas.
It says unless the situation is "kept under control" misbehaving visitors will "irreparably damage the preservation of historical and art areas and monuments and the possibility to enjoy them."
The ban, passed on July 10, began this weekend and stays in effect until the end of October.
Rome - which also passed a crackdown on street vendors - is the latest Italian city to take steps to protect its monuments and limit the effects of mass tourism.
Venice banned picnics in public places and bare torsos in St. Mark's Square; Florence is clamping down on squeegee men who wash the windshields of idling cars and demand payment.
Some tourists lamented that the Roman ban had not been posted and pointed out that there were viable alternatives for tourists who want to avoid the expensive cafes that tack on a surcharge for their outdoor tables.
"You don't want to sit at that place," said Kristin Benner, pointing at one of the expensive cafes near the Pantheon. "And if you have signs, police and benches, isn't that taking away from the monuments more than drinking near them?"
"It's just another way to rip tourists off," said the 22-year-old student from Annapolis, Md.
Bruce Armstrong, a 50-year-old architect from Chicago who was traveling to the Italian capital with his wife and three children, said the emphasis should be on preventing litter, with stiff fines for offenders and more trash cans.
"But if they don't allow tourists to have, say, a cappuccino, a gelato or a sandwich near a monument, that's unfortunate," Armstrong said.
It's still unclear whether police in Rome will be able to enforce the anti-snack measure, given the abundance of artistic sites in the city and its summer influx of tourists. In the first five months this year, at least 7.6 million people visited Rome.
City official Davide Bordoni said police will have to use their judgment in deciding when to intervene. "It is obvious that some situations must be tolerated," he told AP Television News.
So far, police have patrolled sites such as the Spanish Steps, preventing tourists from sipping drinks while sitting on the 18th-century stairway that is a symbol of the city. Other areas in the center of Rome seemed largely unpatrolled.
According to the Corriere della Sera daily, three Tunisian men eating and drinking beer on the Spanish Steps were among the first to be fined.