Jennifer Aniston, Reese Witherspoon and more helped the actor celebrate Saturday
Credit: (L-R)Jeff Kravitz/FilmMagic; C Flanigan/Getty(2); Samir Hussein/WireImage; Jason Merritt

Robert Downey Jr. turned the big 5-0 Saturday, and he had an ample assortment of A-list pals on hand to celebrate the milestone birthday.

Jennifer Aniston and fiancé Justin Theroux, Reese Witherspoon and husband Jim Toth, Gwen Stefani and Gavin Rossdale, Orlando Bloom, Gwyneth Paltrow, Katie Holmes and other famous friends gathered at Santa Monica’s Barker Hangar for Downey’s star-powered soirée.

“It was the best party. Star-studded, of course, but at the same time, it felt very intimate,” a source tells PEOPLE of the bash, where Jeremy Renner, Dustin Hoffman, Edward Norton and Jason Bateman rounded out the impressive guest list. “Everyone seemed friendly and hugged each other left and right.”

Joined by his wife Susan, the Iron Man star and his guests – many of whom showered him with gifts – enjoyed jazz music at the historic event space, where cocktails and hors d’oeuvres were served.

While Holmes showed up sans sometime beau Jamie Foxx, the fête did serve as a date night for many of the couples: Paltrow and was spotted getting cozy with reported love interest Brad Falchuk, while engaged pair Aniston and Theroux shared some sweet P.D.A.

“Jen and Justin looked happy and were very affectionate,” the source adds. “They kept hugging and kissing.”

As for the birthday boy, he took to Twitter, posting a succinct reflection Saturday: