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The couple, who met in 2006, exchanged vows Saturday in front of about 75 friends and family members at his Beverly Hills home.
"The ceremony was very beautiful and romantic," a source tells PEOPLE.
The bride, 31, wore a white dress with her hair pinned up while Williams, 36, wore a black tuxedo.
"At last, Rob is happy," the singer's father, Pete Conway, told Britain's News of the World. "That's what you want for your kids and this is the happiest I've seen him. Ayda is so beautiful and so delightful and so lovely and just so right for him. They are brilliant together."
After the 30-minute ceremony, guests enjoyed a sunset cocktail reception with champagne and gourmet hors d'oeuvre.
The couple will continue their celebration with sit-down dinner and dancing.
Reporting by PERNILLA CEDENHEIM