VENICE, Italy (AP) -- It's a matter of perspective. From aboard a 12-deck cruise liner, the sight of St. Mark's Square, the Doge's Palace and Bridge of Sighs gliding past from a cabin balcony is a breathtaking thrill.
But against the backdrop of Venice's storied canals and Byzantine architecture, these floating condominiums present a jarring sight, out of scale and sync with the surroundings.
The fatal grounding of the Costa Concordia off the Tuscan coast has sharpened the focus on the largely unchecked boom of these ever-larger luxury liners, and nowhere more so than in Venice, a fragile city already struggling against mass tourism and the steady deterioration of its underwater foundations.
There's growing clamor for an urgent rethink to the expanding cruise liner traffic through Venice's historic center. Critics point not only to a threat of accidents, but also air and water pollution, and the injection of an additional 2 million more tourists a year into a city already under constant siege.
The city wants to reroute cruise ships arriving in Venice so they stay farther from St. Mark's and other prominent monuments as a possible step toward keeping them out of the lagoon altogether. And UNESCO, the U.N. culture organization, charges that the liners cause water tides that erode building foundations and pollute waterways.
Venice in the space of 15 years has become one of the world's most important cruise destinations, the port serving as a lucrative turnaround point for 650 cruises a year -- up to nine a day in high season. Since 1997, the number of passengers cruising through Venice has risen from 280,000 to 1.8 million last year.
"One third of all cruise ships worldwide come to Venice each year," said Roberto Perocchio, managing director of the Venice passenger terminal, which manages cruise traffic in the lagoon.
He expects the appetite for cruising to continue to grow, citing forecasts that the 5 million annual cruise passengers a year in Europe is expected to double by 2020.
If the cruise ships were modern buildings, which they strongly resemble, they would certainly not be allowed in Venice, a UNESCO heritage site that mandates the view of protected places cannot be permanently altered. But because they move, there is no official sanction against them.
Just a day after the grounding of the Costa Concordia, dozens of protesters demonstrated as an MSC cruise liner sailed across the St. Mark's basin. Grassroots opposition to the luxury liners predates the Costa disaster, but it has taken new impetus.
"Venice is too often on its knees in front of the gods of economy and tourism, and we have been paying the consequences for years. The city has been emptied of its residents, and it's a victim of pollution from this unsustainable traffic," said Saverio Pastor, a craftsman who makes oarlocks for gondolas, who has been leading the campaign to rid the Venice basin of cruise ships.
Studies commissioned by the Port Authority show that the cruise liners are responsible for up to 30 percent of the city's air pollution.
Residents along the Giudecca canal, the wide waterway through which the ships pass en route to their berths, complain both about the noise and the impact of water being pushed up into the narrower feeder canals, eroding foundations as the water surges and recedes.
"One woman told me that when the ships pass, water from the canals backs up into her toilet," said Jane da Mosto, a longtime Venice resident and scientist for the London-based Venice in Peril association.
The preservation of Venice is an age-old dilemma. In the 15th century, Venetians decided to reroute the rivers emptying into the lagoon to maintain Venice's seafaring course and prevent the waterways from silting up.
"Today, we have the opposite problem," da Mosto said. "The lagoon is being drastically eroded, thanks to the navigation channels that suck sediment out in the wake of the ships. There is now less than one-third of the original salt marsh."
The ships pass within 300 meters (1,000 feet) of St. Mark's Square, but officials for the Venice Port Authority say there is little risk of an accident like the Concordia. For one thing, there are no rocky outposts in the lagoon's muddy bottom. More significantly, they point out that size of the cruise ships requires them to move along deep trenches dredged in the canal bed, leaving little room for disaster.
They cite an incident in 2004, when the 655-foot cruise ship Mona Lisa ran aground near St. Mark's Square in Venice in thick fog. None of the 1,000 passengers and crew on the Mona Lisa was hurt, and port officials say the accident is an example of how Venice can ward off danger.
"It may look like the lagoon is open, and that the cruise ship can go where it wants. But in fact, Venice protects itself," said Ciro Romano, head of the team of 25 captains assigned to board cruise ships outside the lagoon and oversee their passage through Venice, accompanied by a pair of tug boats.
Opponents of the large ships point to another incident in the 1980s, when a container ship went aground on the sidewalk near the park that hosts the Biennale contemporary art show every two years.
To deal with the air pollution, the port is studying a system that would permit the ships to plug into a power grid when docked, allowing them to turn off their engines. But Pastor said the system has long been under study and will require huge investments.
Port officials also say that there is no question of informal salutes like the one that drew the nearly 300-meter Costa Concordia close to Giglio island. But critics charge that every time a boat passes St. Mark's it is in effect, a salute, even if the horns don't sound.
Even before the Concordia disaster, Venice Mayor Giorgio Orsoni and the chief port official signed an agreement for new studies on alternative routes -- but passenger terminal officials believe that passing by St. Mark's is a key attraction.
Francesco Bandarin, UNESCO's assistant director-general for culture -- and a Venetian himself -- said longer-term solutions are needed.
"The city is a very fragile city. This is a city that comes to us from the Middle Ages," Bandarin said. "It is not designed for having that kind of traffic. It is designed to have ships, and we will always have ships around Venice, but not these kind of ships."