Actor-comedian Robin Williams has been found dead at his home in northern California.
In a statement, Marin County Sheriff's Department said officers had been called to the 63-year-old's house at 11:55am on Monday (local time) after he was found unconscious and not breathing.
He was pronounced dead shortly afterwards. The Sheriff's Department said an investigation into his death was underway, but that it appeared to be suicide.
The Oscar-winning actor was known for roles in acclaimed films such as Good Morning, Vietnam (1987), Dead Poets Society (1989), Good Will Hunting (1997), Mrs Doubtfire (1993) and Jumanji (1995).
Williams' widow, Susan Schneider, released a statement, saying: "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."
Williams struggled with depression and addiction and entered a Minnesota rehabilitation centre last month.
His representatives at the time said Williams was not using drugs or alcohol but had gone to the centre to "fine-tune and focus" his sobriety after working a longer-than-usual schedule.
"Robin Williams passed away this morning. He has been battling severe depression of late," publicist Mara Buxbaum said in a statement shortly after his death.
"This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."
Born in Chicago in 1951, Williams rose to fame with his role as the alien Mork in the TV series Mork and Mindy.
He went on to establish a career in both stand-up comedy and film action, and also did voice work, including on Aladdin and Happy Feet.
Williams is scheduled to appear in Night at the Museum: Secret of the Tomb on December 19, 2014, reprising his role as the statue of Teddy Roosevelt who comes to life at night.
Nominated for three Academy Awards, he won the best supporting actor Oscar for his performance in Good Will Hunting.
He also received two Emmy Awards, four Golden Globes, two Screen Actors Guild awards and five Grammys.
The actor had described himself as a quiet child who overcame his shyness after becoming involved in drama during high school.
In 1973, Williams became one of only two students accepted into the advanced program at the Julliard School - the other being Christopher Reeve.
After his work on Mork and Mindy in the late 1970s, Williams attracted a wider audience with his stand-up comedy, a consistent thread throughout his career.
Known for his improvisational skills and impersonations, most of his career focused on the silver screen.
His first film was the 1977 comedy Can I Do It 'Till I Need Glasses? His performance in Good Morning, Vietnam earned him an Academy Award nomination.
His unique voice brought the genie to life in Aladdin (1992), with Williams going on to voice parts in Fern Gully (1992), Robots (2005) and Happy Feet (2006).
He earned critical acclaim for his roles as an English teacher in Dead Poets Society, and later as a psychologist in Good Will Hunting, alongside Matt Damon and Ben Affleck.
His other films include Hook, What Dreams May Come, and thrillers Insomnia and One Hour Photo.
Williams was married three times, and has three children. He lived with his third wife in San Francisco until his death.
Legendary director Steven Spielberg paid tribute to his close friend.
"Robin was a lightning storm of comic genius and our laughter was the thunder that sustained him," Spielberg said in a statement cited by Variety.
"He was a pal and I can't believe he's gone," added Spielberg, who famously phoned Williams to cheer himself up during filming of his harrowing 1994 Holocaust drama Schindler's List.
Spielberg's tribute was echoed throughout the entertainment industry.
"I can't believe the news about Robin Williams. He gave so much to so many people. I'm heartbroken," comic and talkshow host Ellen DeGeneres said on Twitter.Obama pays tribute to 'one of a kind'
President Barack Obama joined the outpouring of grief.
"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," Mr Obama said.
"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."
Williams struggled with drug addiction during the late 1970s and early '80s. He credited the death of his friend, fellow actor John Belushi, and the birth of his first son with helping him quit drugs.
"Was it a wake-up call? Oh yeah, on a huge level. The grand jury helped too," he told Inside the Actors Studio in 2001.
In 2006 his publicist announced the actor had "found himself drinking again" and checked into a rehab centre.http://www.abc.net.au/news/2014-08-12/robin-williams-dead/5664414
Truly shocking... my thoughts are with his family...