The couple, who got engaged last month, partied with pals at Cafe Habana, which is owned by Clooney's friend and business partner Rande Gerber.
The globe-trotting pair, who recently flew to Mexico to attend a friend's wedding, were relaxed and affectionate at the celebration.
Guests, including Gerber and his wife Cindy Crawford, as well as U2's Bono and The Edge, gathered on the restaurant's patio, according to a source, who added that several of the guests performed karaoke.
The group toasted the couple with tequila and also sipped wine.
"After the toast, George and his fiancée were all over each other," a source says. "Lots of hugging."
Alamuddin was showing off her emerald-cut diamond engagement ring (estimated at more than 7 carats and which Clooney helped design) at the party, which ended early in the evening.
According to the source, Clooney, 53, and Alamuddin, 36, left in a limo, toting gifts and flowers.
The couple, who were first seen out together in London in October, fell hard and fast for each other, and a friend of Clooney's tells PEOPLE that Alamuddin is a perfect match for the Oscar winner.
"He has found someone who is as smart as he is and who is her own independent woman," the pal says. "Amal seems to be the best fit for him he's ever had."
And there's another reason the British attorney is the ideal woman for the reformed bachelor – "they look good together," says Alamuddin's friend Jean Pigozzi. "She's a very smart and serious girl."