About 90 private jets are set to land in local airports this week, bringing A-listers from show-business, politics and finance to the widely-dubbed "wedding of the century" that will cost Bezos an estimated 40-48 million euros ($A71-86 million).
The couple were spotted by a Reuters reporter entering the exclusive Aman Venice Hotel on the Grand Canal, where many of the celebrities will stay.
Festivities will kick off on Thursday night with an open-air gathering in the cloisters of Madonna dell'Orto, a medieval church in the central area of Cannaregio known for its lively nightlife, a source close to the matter told Reuters.
Venice City Hall issued a directive on Wednesday cordoning off the area, isolating guests from activists who have been protesting for weeks that the celebrations will turn the city of gondolas and palazzi into a private amusement park for the rich.
"There's only one thing that rules now: money, money, money, so we are the losers," Venice resident Nadia Rigo said.
"We who were born here have to either move to the mainland or we have to ask them for permission to board a ferry. They've become the masters."
The date and venue of the wedding itself are still unknown and some have speculated that Bezos, 61, and Sanchez, 55, may already have tied the knot in a private ceremony back in the United States.
Most of the estimated 250 guests are expected to arrive in time for the second party, which the source said is slated for Friday on the small island of San Giorgio, in front of the famed St Mark's Square with its 99-metre-high bell tower.
San Giorgio island is home to the Cini Foundation, which organises international cultural and political meetings in an ancient monastery dating back to the year 1000, with a maze in the garden.
Outside the Aman Venice Hotel, workers erected a striped canvas canopy with side walls on a floating wharf to protect guests from prying eyes and photographers' long lenses.
Celebrations will conclude on Saturday with the main wedding bash to be held at one of the halls of the Arsenale, a vast former medieval shipyard turned into an art space in the eastern Castello district.
Surrounded by water and impossible to reach by land when connecting bridges are raised, the hall is considered a safer site than a previous location choice for the final party, a former medieval religious school in Cannaregio.
Bezos, executive chair of e-commerce giant Amazon and no.4 on Forbes' billionaires list, got engaged to Sanchez in 2023, four years after the collapse of his 25-year marriage to Mackenzie Scott.
The couple's decision to marry in Venice follows other celebrity weddings in the floating city, such as that of US actor George Clooney and human rights lawyer Amal Alamuddin in 2014.
