The loved-up pair arrived hand-in-hand for the Malibu event held at Café Habbana, where they were joined by the likes of Cindy Crawford and her husband, Rande Gerber, who owns the restaurant.
The famous group gathered on the back patio, where they toasted the future bride-and-groom with tequila and wine.
“After the toast, George and his fiancée were all over each other,” a source told People. “Lots of hugging.”
Smiling Amal could be seen showing off her impressive seven-carat diamond engagement ring as she chatted with her husband-to-be’s close friends.
There was even a bit of karaoke; Bono got up and sang during the party, as did supermodel Cindy, who reportedly duetted with her daughter Kaia.
An eyewitness told E! News that the venue was busy with people celebrating Mother’s Day. “They got a show they didn’t expect and other (restaurant patrons) said it was the best Mother’s Day ever.”
George, 53, proposed to British human rights lawyer Amal, 36, in late April, after seven months of dating.
The future Mr and Mrs Clooney recently returned from a holiday in Cabo, Mexico, where they celebrated a friend’s wedding. “You can tell these two are in love,” an onlooker said.