In a tribute to the 63-year-old comic actor's influence on younger celebrities, Williams' co-stars from "Good Will Hunting" -- the film that won him his Oscar -- joined the global outpouring of emotion.
"Heartbroken. Thanks chief - for your friendship and for what you gave the world," said Affleck. "Robin had a tonne of love in him. He personally did so much for so many people. He made Matt and my dreams come true. What do you owe a guy who does that? Everything."
Damon, in a separate statement, said: "Robin brought so much joy into my life and I will carry that joy with me forever. He was such a beautiful man."
Further from the rarefied air of Hollywood, the stand-up performers who shared the comedy circuit with Williams also chimed in.
"Goodbye pal. Thanks for everything," wrote Louis C.K., who cast Williams to play himself in an episode of his sitcom.
"So sad. So funny," added Chris Rock, one of the few stand-ups who could fill an arena-sized venue like Williams.
The doctor and clown whose life story formed the basis of the 1998 movie "Patch Adams," for which Williams earned a Golden Globe nomination, also paid tribute.
"The terrible news of the passing of Robin Williams reached me here in the Peruvian Amazon late last night with tremendous sadness," Hunter Doherty Adams wrote on Facebook.
Williams was also saluted by many military veterans, hailed as one of the most dedicated of the performers to tour overseas war theaters like Iraq and Afghanistan to entertain US troops.
A formal investigation is under way, but the local sheriff's office in Marin County, California said all the evidence points to Williams having taken his own life by hanging himself with his belt.
"His life ended due to asphyxia due to hanging," said assistant coroner Lieutenant Keith Boyd, revealing that Williams' personal assistant had found him slumped in his home near San Francisco on Monday.
"Mr Williams, at that time, was cool to the touch with rigor mortis present in his body," Boyd told reporters, adding that the inside of Williams' left wrist bore superficial cuts.
Boyd refused to say whether a note was found, but a pocket knife with signs of dried blood on its blade is being examined. A cause of death will not be declared until toxicology tests are complete.
Williams -- star of such hit films as "Good Morning, Vietnam" and "Mrs Doubtfire" -- had last been seen alive Sunday by his wife Susan Schneider before she went to bed, he said.
She left the house on Monday assuming he was still asleep in a separate room in the home in Tiburon, north of San Francisco.
Father-of-three Williams had spoken openly in the past about his battles with alcoholism and drug abuse -- and often drew on them in his stand-up comedy routines.
The comedian's daughter, Zelda Williams, wrote in a Tumblr post that while she would "never, ever understand how he could be loved so deeply and not find it in his heart to stay, theres (sic) minor comfort in knowing our grief and loss, in some small way, is shared with millions."
Wrestled with drinking, drugs
President Barack Obama led public tributes to an entertainer he described as "one of a kind" and Hollywood titan Steven Spielberg hailed his close friend as a "lightning storm of comic genius."
Former US president Bill Clinton meanwhile tweeted: "Grateful for the life of Robin Williams, a true talent and a wonderful friend. He will be missed by so many."
Fans worldwide united in grief via social media, with #RobinWilliams trending on Twitter throughout the day.
Miniature shrines popped up at Williams' star on Hollywood's Walk of Fame, a park bench in Boston that was featured in "Good Will Hunting" and the Victorian house in Colorado where his breakthrough 1970s space-alien sitcom "Mork and Mindy" was recorded.
"He was a giant heart, a fireball friend, a wondrous gift from the gods," said British actor, director and Monty Python alum Terry Gilliam. "Now the selfish bastards have taken him back."