Unfortunately, the actor had struggled with substance abuse and addiction throughout his career. Though he was prompted by the birth of his son to quit drugs in the 90s, the comedian has mentioned on several occasions that staying clean requires constant effort. In 2006, he checked himself into a rehabilitation center, later admitting that he was an alcoholic.

Robin Williams Mork Mindy HouseFans leave tributes outside the 'Mork & Mindy' house [Getty/Marc Piscotty]

Last month Mr. Williams checked into an addiction centre in Minnesota for several weeks.

"After working back-to-back projects, Robin is simply taking the opportunity to fine-tune and focus on his continued commitment, of which he remains extremely proud," his spokesperson said at the time, according to TMZ.

Some of the tributes focused on his professional legacy, like Barack Obama’s statement, which read: "He was one of a kind. He arrived in our lives as an alien - but he ended up touching every element of the human spirit. He made us laugh. He made us cry.

"He gave his immeasurable talent freely and generously to those who needed it most - from our troops stationed abroad to the marginalised on our own streets."

Others remembered Mr. Williams as a friend more than anything else.

Pam Dawber, Williams' co-star in Mork and Mindy, the television show that catapulted him to fame, said: "I am completely and totally devastated."

John Travolta, his co-star in the film Old Dogs, said: "I've never known a sweeter, brighter, more considerate person than Robin. Robin's commitment as an artist to lifting our mood and making us happy is compared to none. He loved us all and we loved him back."

Mia Farrow, the actress, tweeted: “No! Robin Williams you were so loved.”

David Steinberg, his manager of 35 years, said: "Nobody made the world laugh like Robin Williams. My brother, my friend, my soulmate, I will miss you."

More: Robin Williams Was Set To Reprise His Mrs. Doubtfire Role In A Sequel

His daughter Zelda, who recently celebrated her 25th birthday, later posted an excerpt from French poet and aviator Antoine de Saint-Exupery's El Principito in tribute.

She added: "I love you. I miss you. I'll try to keep looking up."