Brad Pitt and Jennifer Aniston, who split five years ago, were both on the same soundstage at L.A.'s CBS Studios where he presented and she manned the phones at the Hope for Haiti Now: A Global Benefit for Earthquake Relief, but they did not cross paths, according to a source.
Justin Timberlake, who sang a soulful rendition of "Hallelujah" with Tennman artist Matt Morris, bumped into his ex, Cameron Diaz, off camera. "They ran into each other after he performed and she was in the phone bank and then they talked," the source says.
"The celebrities were so low-key. Everyone drove themselves, there were no handlers, no entourages," adds the source. "Everyone was in a good mood. Most people showed up early and many celebrities, including Chevy Chase, Olivia Wilde and David Archuleta, stayed after the telethon was over so they could keep answering phone calls.
The A-list event was co-hosted by George Clooney and Wyclef Jean and presented by MTV Networks and broadcast on nearly a dozen channels.
Making Connections"Celebs were catching up with each other and sitting down with staff and just chatting, talking about how sad the situation in Haiti was and how happy they were to help. The success of the night put a lot of people in a euphoric mood and a lot of people made plans to get together later," the source adds.
Before the show started, early arrivals Reese Witherspoon and Julia Roberts were deep in conversation when Drew Barrymore came over to join them before all three women took their seats at the phone bank.
Penelope Cruz approached organizer Clooney, who a source says was "very hands-on" during the telecast, and told him, "You're a genius," after he brought so many people together for the event, which raised money by encouraging viewers to call 1-877-99-Haiti to pledge money for the cause.
Backstage: Newlywed Kevin Jonas ran into Zac Efron's girlfriend Vanessa Hudgens and expressed his hope that "the four of us" would hang out soon.
In the greenroom, Chris Rock noshed on a cupcake from Crumbs bakery while talking to Ben Stiller. Morgan Freeman and Halle Berry chatted about her daughter Nahla. And Leonardo DiCaprio drank white wine while Archuleta rejoiced when he found papaya on the fruit platter.
Christian Slater rocked out while watching Madonna's performance of "Like a Prayer" via satellite from New York; Pussycat Doll frontwoman Nicole Scherzinger and Rose McGowan were inseparable most of the night; Rande Gerber dutifully waited for wife Cindy Crawford in the green room while she worked the phones.
"It was a great night," says the source. "It was surreal to see so many celebrities together in one place."
After-PartyFor many of the stars, the fun continued at L.A.'s Tower Bar at the Sunset Tower Hotel where Clooney, DiCaprio, Witherspoon, Daniel Craig, Kid Rock, Aniston and Gerard Butler stopped by for some dinner and cocktails after the telethon.
"Everyone seemed really happy, almost giddy," an onlooker tells PEOPLE. "They were obviously celebrating the huge amount of money that they raised, and cutting loose at the same time."
With reporting by REAGAN ALEXANDER