Actor Robin Williams has been found dead in what police said was a suspected suicide at his home in California.
Williams, 63, known for films such as Good Morning, Vietnam and Mrs Doubtfire, had been grappling with severe depression recently, according to his press representative.
In a statement his wife Susan Schneider said: "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings.
"I am utterly heartbroken. On behalf of Robin's family, we are asking for privacy during our time of profound grief. As he is remembered, it is our hope the focus will not be on Robin's death, but on the countless moments of joy and laughter he gave to millions."
Mara Buxbaum, Williams' press representative, said: "Robin Williams passed away this morning. He has been battling severe depression of late.
"This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."
A Marin County Sheriff's Office statement said: "At this time, the Sheriff's Office Coroner Division suspects the death to be a suicide due to asphyxia, but a comprehensive investigation must be completed before a final determination is made."
It added that an investigation into the "cause, manner, and circumstances of the death is currently under way".
A forensic examination is expected to take place today, with toxicology tests to take place later.
His daughter Zelda Williams, who recently celebrated her 25th birthday, posted an excerpt from French poet and aviator Antoine de Saint-Exupery's El Principito on Twitter, which read: "You - you alone will have the stars as no one else has them... In one of the stars I shall be living. In one of them I shall be laughing. And so it will be as if all the stars were laughing, when you look at the sky at night ... You - only you - will have stars that can laugh."
She added: "I love you. I miss you. I'll try to keep looking up."
Police were called to Williams' home in the town of Tiburon at around 11.55am local time yesterday (7.55pm BST) after receiving a call that a man was unconscious and not breathing.
Emergency services including police and firefighters arrived on the scene five minutes later, but the actor was pronounced dead at 12.02pm, the sheriff's office said.
It added that Williams was last seen alive at his home, where he lived with Ms Schneider, at around 10pm on Sunday.
Williams shot to fame in the late 1970s as an alien in the American TV comedy series Mork & Mindy. But he won his Academy Award in a serious role, as the therapist in Good Will Hunting.
US president Barack Obama led tributes to the star as a host of Hollywood A-listers spoke of their shock at Williams' death.
He said: "Robin Williams was an airman, a doctor, a genie, a nanny, a president, a professor, a bangarang Peter Pan, and everything in between. But 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.
"The Obama family offers our condolences to Robin's family, his friends, and everyone who found their voice and their verse thanks to Robin Williams."
Steven Spielberg, who worked with Williams on Hook, said: "Robin was a lightning storm of comic genius and our laughter was the thunder that sustained him. He was a pal and I can't believe he's gone."
Steve Martin, who appeared alongside Williams in a 1988 theatre production of Waiting for Godot, said: "I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul."
Williams' Good Will Hunting co-star Minnie Driver said: "My heart's broken. Robin was a beautiful, kind soul. Can't bear that he's gone. So incredibly sorry for his family."
Talk show host Ellen DeGeneres tweeted: "I can't believe the news about Robin Williams. He gave so much to so many people. I'm heartbroken."
Graham Norton, who had interviewed Williams on his BBC chat show, said: "Such sad news. My heart goes out to his family. The great Robin Williams."
Jonathan Ross, who had also interviewed the actor, said: "Robin Williams is gone. I loved him. Incredibly talented performer. Wonderfully sweet and generous person. Heartbreaking news. So sad.
"What a devastating loss."
Police are expected to release more information about his death at a press conference scheduled for 7pm British time today.
Kevin Spacey, who was in the 2009 film Shrink alongside Williams, tweeted: "Robin Williams made the world laugh & think. I will remember & honor that. A great man, artist and friend. I will miss him beyond measure."
Actress Mara Wilson, who played Williams' daughter in Mrs Doubtfire, said: "Very sad, very upset, very glad I did not have to hear about this through Twitter. Probably going to be taking some time off it for a while."
Read Robin Williams' memorable roles here
Read celebrities pay tribute to Williams here