The future Mr. and Mrs. Clooney were joined by some of their famous friends, including U2 frontman Bono, the restaurant's owner Rande Gerber and his supermodel wife, Cindy Crawford.
Sources told E! News the low-key celebration was well underway around 7 p.m.
Clooney looked relaxed and was dressed in a casual blue shirt with jeans. His human rights attorney wife-to-be wore a green T-shirt with floral shorts
One eyewitness said the restaurant was very busy because of the Mother's Day crowd.
"People got a show they didn't expect and other [restaurant patrons] said it was the best Mother's Day ever," the source exclusively told E! News.
It's been a whirlwind past few days for the happy couple. They were photographed together Wednesday night in Cabo San Lucas.
A source exclusively told E! News the Monuments Men star, who just celebrated his 53rd birthday, and his stunning fiancée, 36, had a romantic dinner followed by an evening of fun with nine friends at downtown hot spot Harbard. The pair were holding hands throughout the night, and as one eyewitness puts it, "You can tell these two are in love."
Clooney popped the question to Alamuddin last month.
This will be the second trip down the aisle for the Oscar-winning actor, who was married to Talia Balsam from 1989 to 1993—after which he vowed never to do such a thing again. Or so the Hollywood legend goes.
—Additional reporting by Senta Scarborough and Michelle Falls