Comedian and actor Billy Connolly, a close friend of Williams, said: "Robin was both my friend and my hero, a unique talent and a kind and generous man; the world will be a much poorer place without him."
Eddie Izzard tweeted: "Robin Williams has died and I am very sad. From every comedian here at the Edinburgh Fringe Festival, we salute him & we say goodbye."
Steve Carell, star of The 40-year-old Virgin and the US version of The Office, said: "Robin Williams made the world a little bit better. RIP."
Comedian Jason Manford tweeted: "RIP Robin Williams. So unbelievably sad about this news. One of the funniest talented comedians the world has ever produced."
DJ Chris Moyles remembered Williams's breakthrough role in Mork & Mindy when he tweeted: "Rest in peace Robin Williams. Nanu Nanu. Shazbot."
Anthony McPartlin, one half of presenting duo Ant & Dec, said: "Robin Williams RIP. X My childhood was made very happy because of him. Thoughts and love to his family. So so sad."
Phillip Schofield spoke of his shock when he tweeted: "Oh my god! Can't believe Robin Williams has died in an 'apparent suicide' tragic news and the loss of a comedy genius."
X Factor judge David Walliams said he had the "huge pleasure" of spending time with Williams in Los Angeles with fellow Little Britain star Matt Lucas.
He said on Twitter: "I was so in awe of him but he was so sweet and humble.
"We saw Robin Williams perform unannounced at comedy clubs in LA. His talent was breathtaking & I have never seen such love from an audience."
Family Guy creator Seth MacFarlane said: "The world just got a lot less funny. Robin Williams is a tragic loss."
And comedian Simon Pegg called him a "true great", posting a YouTube link to a scene from Good Will Hunting.
Stephen Fry said: "Devastating news about @robinwilliams - knew him a little and liked him a whole lot more. A brain wired like no other and so so kind."
And Rory Bremner said: "Really saddened to hear Robin Williams has died. He was a Catherine wheel of comic energy - brilliant, irrepressible, inspired. A true genius.
"His imagination was like electricity - flowed through him, lit him up, sparked wildly, consumed him. He inspired my generation."
Film director Ron Howard recalled that he saw Mork's genesis as they worked together on the US series Happy Days.
In a message posted online, he said: "We lost Robin - first witnessed his genius as he created Mork before our eyes in two hours on set. A Force. A Sweet Soul. A Brilliant Artist."
Comedy film-maker Judd Apatow said: "Isn't it mind-blowing how many times and ways Robin Williams touched your life with his work? For so long. What a gift he gave."
Omid Djalili recalled that Williams mistook him for Alexei Sayle each time they met. "Had the pleasure of Robin Williams' company five times. Each time he exclaimed 'Alexei!" and gave me a hug. I never corrected him," he said.
Hollywood actor Michael Keaton said: "Just so damn sad. This guy made so many people happy. There will always be that."