The world's most famous bachelor, George Clooney, turned Venice's Grand Canal into an aquatic red carpet on Saturday as star-studded festivities began in earnest for his wedding to human rights lawyer Amal Alamuddin.

The Italian city of gondolas and palazzi looked like Hollywood on the Adriatic as A-list guests cruised between luxury hotels for the extravaganza, which has been billed as the party of the year even as details were kept secret.

Actors Matt Damon and Bill Murray mingled with model Cindy Crawford, Vogue editor Anna Wintour and singer Bono at a reception on the terrace of the plush Cipriani hotel ahead of a gala dinner on Saturday night.

A tuxedo-clad Clooney helped guests onto water taxis at the Cipriani before heading across the lagoon for the main party at the seven-star Aman Hotel in the 16th-century Palazzo Papadopoli on the Grand Canal.

Passing "vaporetti" public boats tilted sideways as passengers crowded to see the two-time Oscar winner, who waved to onlookers and rubbed his hands together on arrival at the Aman.

Drinks were laid on in a rear garden before dinner in the main dining room, where the pair were to say vows informally ahead of a civil service on Monday, according to local reports.

Kentucky-born Clooney, who shot to stardom in the television series "ER", had vowed never to remarry after his 1993 divorce from actress Talia Balsam and is said to have made a $100,000 bet with Michelle Pfeiffer that he would stay single.

The Lebanese-born Alamuddin, who had no previous connection to Hollywood, has represented Ukrainian former prime minister Yulia Tymoshenko at the European Court of Human Rights, and WikiLeaks founder Julian Assange in extradition proceedings.

Alamuddin, who is based in Britain, also advised former United Nations secretary general Kofi Annan on the conflict in Syria, an issue about which Clooney has spoken publicly.

Clooney has also led campaigns to highlight the plight of refugees in Darfur, Sudan, and is expected to donate the fees earned from selling rights to the wedding photos to that cause.

WHAT BETTER SPOT?

As the sun shone over Venice, locals and tourists alike were excited to witness a memorable moment in the city's long history of hosting stars for its film festival, the world's oldest.

"I thought he was like the world's most eligible bachelor so I didn't realize he was ever going to get married, but I guess if you're going to get married, what better spot than a beautiful Italian city like Venice?" said Canadian tourist Philip De Vooght.

"She will keep him on his toes because she is bright," said Judith Graham, 59, from Surrey, England. "He's had all these actresses fawning over him before. I hope he'll be happy."

A slick, wooden speedboat called "Amore" brought the couple into the city on Friday in a convoy of boats, a modern-day version of the ceremonial entrances of the Doges of the Venetian Republic, which lasted from the late seventh century until 1797.

Alamuddin, who is rumored to have 12 outfit changes prepared for the weekend, wore a black-and-white striped dress as she arrived with Clooney on the speedboat.

The 36-year-old picked a more formal red dress with black butterfly patterns for a party in her honor on Friday night. She has reportedly chosen an Alexander McQueen wedding dress.

Guests were ferried around the city in speedboats bearing small flags marked with "AG", for Amal and George, and a brown swagged canopy was put up in front of the Aman Hotel on Saturday to shield guests from prying eyes and photographers' long lenses.

The official ceremony is expected to be held on Monday at Venice's town hall, the 14th-century Ca' Farsetti palace. Italian media reported that the former mayor of Rome, Walter Veltroni, a film buff and friend of Clooney, would officiate.

Venice's city government said it would close off a few of the town's narrow pedestrian streets for two hours on Monday.

"Considering that the location of the ceremony is likely to become a target for people attracted by the celebrity status of the event, high numbers could be a problem for traffic and pose a threat to those people's safety," the local government said.