The wedding began at 6:30 p.m. in the Long Island, New York, town of Amagansett. The ceremony took place under a gazebo built for the occasion, with guests sitting on white benches.
Guests included Keanu Reeves, who arrived by motorcycle, Billy Joel and Sting.
The reception followed at the same location, with tables decorated in red and gold and topped with exotic greenery instead of flowers.
At lunch, Downey didn't eat much. "He just drank a lot of water," adds the friend. "He was too nervous."
The rehearsal dinner Friday night was held at the East Hampton Point Restaurant, hosted by Levin’s parents.
Downey, 40, and Levin, 31, met in 2002 on the set of Gothika, the psychological thriller he made with Halle Barry, which Levin coproduced.
The couple were engaged in November 2003 when Downey presented Levin with an engagement ring made of diamonds and a rare African sapphire.
Downey – who has starred in more than 50 films including Less Than Zero, Chaplin and Wonder Boys – has a reputation as a talented but troubled actor. In June 1996 he was pulled over for speeding, and police found heroin, crack and an unloaded gun in his car. He spent the next six years in and out of prison and rehab.
Downey, who has an 11-year-old son, Indio, with ex-wife Deborah Falconer, however, has turned his life around, and has been sober since meeting Levine.
"She's been very firm that if he's going to be with her, he has to really toe the line, and he's done that," Gothika producer Joel Silver told PEOPLE in 2003.
It's a sentiment Downey Jr. shares – with his characteristic flair: She "has got me in a freakin' choke chain," he told PEOPLE at the time, "and I love it."