Amazon tycoon Jeff Bezos and Lauren Sanchez tied the knot on Friday at a sumptuous ceremony with the rich and famous on an island in Venice's lagoon, out of the sight and sound of protesters.

"This city seems impossible! It can't exist and yet, here it is!" an enchanted Bezos told a La Repubblica journalist on Thursday who got close to the magnate as he whizzed around the canals by boat.

But protesters had a different view, wondering how long Venice can endure: While the billionaires party, activists say the fragile city is sinking, overrun by tourists, and a victim of depopulation as locals unable to pay soaring rents are forced out.

"No Kings, No Bezos" read a sign in green neon projected on the St Mark's Campanile tower on Thursday night.

Another sign read: "In the time it takes you to read this, Jeff Bezos's wealth has increased by more than your monthly salary."

Bezos and Sanchez, a former news anchor and entertainment reporter, celebrated their nuptials with guests including Kim and Khloe Kardashian, Oprah Winfrey and Orlando Bloom.

The tech magnate, 61, and Sanchez, 55, are staying at the Aman hotel, a luxury 16th-century palazzo on the Grand Canal with a view of the Rialto bridge. Other A-list guests are staying at the Gritti Palace and the St. Regis.

The couple exchanged vows at a black-tie ceremony on the island of San Giorgio Maggiore Friday afternoon, according to Italian media reports.

The wedding is thought to have taken place in a vast open-air amphitheatre on the island, which sits across from Venice's iconic St Mark's Square. (AFP)