VENICE—Details of the wedding of Jeff Bezos and Lauren Sánchez are a tightly kept secret. That isn’t stopping some angry Venetians, armed with buoys and motorboats, from plotting to disrupt it.

“We will be in the canals, we will be in the alleyways and we will be on the waterfront,” said Na Haby Stella Faye , delivering her vow of Churchillian defiance to around 200 Venetians gathered in a small piazza. “No Space for Bezos,” read a banner behind her, alluding to the American magnate’s space venture .

The goal, she told the cheering crowd: “To stop this wedding!”

The Amazon founder and his fiancée are expected to tie the knot in Venice this coming week in a lavish, star-studded, multiday event that will take place across the city’s tiny islands. Many Venetians aren’t enthused.

People at a demonstration. Photographs by Clelia Cadamuro for WSJ

They are already exasperated by a crush of tourists and doubly fed up with the world’s superrich using the lagoon as a backdrop.

“It’s absurd to treat this city like it’s Disneyland,” said Grazia Satta , a retired teacher and social worker. “The message this wedding sends is that rich people can do whatever they want. We shouldn’t kneel before wealth like this.”

For Venice’s local government, however, getting high-net-worth individuals to spend time and money here is precisely the point. The mayor’s office encouraged the celebrity couple to marry in the city because it would bring lucrative work to everyone from florists to dockhands.

“These are not tourists who come for a couple of hours and then leave with a sandwich in their hands,” said Simone Venturini , the city councilor in charge of tourism. The municipality is hoping the event could lead to investments by Bezos or one of his wealthy guests.

Venturini dismissed the anti-Bezos activists as attention-seeking radicals, but he acknowledged their protest could have a real impact. The biggest fear is that Bezos and Sánchez could reconsider marrying here at the last minute.

“If Bezos’ wedding goes ahead as planned, without these pain-in-the-ass protests, Venetian citizens won’t even notice,” Venturini said.

A participant at a ‘No Space for Bezos’ event in Venice. Photographs by Clelia Cadamuro for WSJ

The wedding organizers, the London-based firm Lanza & Baucina, said its clients are committed to minimizing disruption during their celebrations.

“Rumours of ‘taking over’ the city are entirely false and diametrically opposed to our goals and to reality,” the company said. “We have always acknowledged the wider debate and critical issues surrounding the city’s future, and from the outset our client has been honoured to support the city and its all-important lagoon through non-profit organizations and associated projects.”

Jeff in Venice

Exactly when and where the wedding is taking place isn’t publicly known. Virtually everyone involved has signed nondisclosure agreements. But Venice is small, and rumors travel fast.

“We have our moles,” said Federica Toninello , a young activist planning to interrupt the celebrations.

Italian media initially reported the festivities would start June 24. Activists said those dates were misinformation and that celebrations would actually begin June 26.

Their main target is one of the rumored venues: the Scuola Grande Della Misericordia, a vast, Renaissance-era brick building with frescoed walls in the heart of Venice. It is surrounded by canals on three sides. Activists plan a waterborne protest on June 28.

Wedding organizers say they have booked 30 of the city’s roughly 280 water taxis.

None of the gondoliers approached by a Wall Street Journal reporter said they were aware of gondolas being booked for the big event. “We are too slow,” one said dolefully.

Bezos and Sánchez are just the latest VIPs to choose Venice for their wedding party. George and Amal Clooney were married here in 2014. A son of Indian billionaire Mukesh Ambani held extravagant pre-wedding celebrations here last year.

Many people in Venice blame Amazon for forcing shops to close, a problem felt acutely in a city where stores that cater to residents often struggle to stay open.

“I remember George Clooney’s wedding and honestly it didn’t bother me. But this is different,” said Irene Boscolo , who runs a creperie along a Venice canal. “Bezos is a genius but he is on the wrong side.”

A general view of the exterior facade of seven star hotel, Aman Resort, where the guests of US actor George Clooney and Amal Alamuddin are arriving to a attend their allegedly wedding party in Venice, Italy, 27 September 2014. EPA/ANDREA MEROLA

Where’s the exit?

Venetians have an ambivalent relationship with tourism. The city’s economy entirely depends on it. Most days, tourists easily outnumber the resident population, which has declined from around 175,000 in the 1950s to below 50,000 today.

To tackle overtourism, the local government last year introduced a daily entry fee of €5 or €10 on busy days, equivalent to around $5.75 or $11.50. It has done little to dissuade visitors from descending en masse on La Serenissima .

Some residents complain that the entry ticket only reinforces the perception that Venice is a theme park.

Some tourists literally think it is, asking astonished locals questions such as, “Where is the exit?” and, “What time does it close?”

As more apartments are converted into short-term rentals, locals say finding affordable housing is the single biggest obstacle to staying.

“This is a city that is losing its identity, it’s losing its soul because it has become so difficult to live here,” said Mattia Berto , a theater director whose plays have addressed Venice’s disintegrating social fabric.

“Venice has always been a stage-like city,” said Berto. “But I would also like it to be a stage for everyday life.”

The activists have caught the attention of visitors. After last week’s protest, they hung their massive “No Space for Bezos” banner from the Rialto Bridge. From the waterfront below, amused tourists snapped photos with their smartphones.

“What are they protesting about?” one American tourist asked another. “Bezos,” said the other, chortling.

Write to Margherita Stancati at margherita.stancati@wsj.com