Due to privacy concerns, no official information is available about the four-day fest, which starts on Friday night. Unconfirmed reports suggest that Cate Blanchett, Matt Damon, Bono, Cindy Crawford, Brad Pitt and Angelina Jolie are among the attendees.
The 36-year-old Alamuddin, who is normally based in London, was spotted at Milan airport on Wednesday, while there have been no sightings of Clooney so far. The bride-to-be held a garment bag from the Stella McCartney fashion label, possibly containing her wedding dress.
Italian tenor Andrea Bocelli and US pop star Lana Del Rey are expected to provide musical entertainment, the Corriere del Veneto newspaper wrote, adding that most celebrations will take place in the five-star Belmond Hotel Cipriani.
However, guests will be moved to the seven-star Aman Canal Grande - a 16th century palazzo overlooking Venice's main waterway - for dinner on Saturday, partly because the Cipriani had already been booked for another wedding, Corriere del Veneto said.
It will be Clooney's second wedding, after his 1989-1993 union with actress Talia Balsam. He is often seen in Venice - usually to attend the lagoon city's film festival. He also owns a villa overlooking Lake Como, near Milan.
The 53-year-old is said to have asked former Rome mayor and ex centre-left leader Walter Veltroni to officiate a civil wedding ceremony on Monday. Crawford's husband Rande Gerber is expected to be the best man, while Alamuddin's sister Tala will be the maid of honour.
The Municipality of Venice was not available for comment.
Clooney had announced plans to tie the knot in Venice earlier this month.
At a charity gala in Florence, he said: "I would just like to say to my bride-to-be Amal that I love you very much and I can't wait to be your husband. I met my bride-to-be in Italy, and I will be married in Italy soon, in a couple of weeks ... in Venice, of all places."