By Lauren Raab, Ryan Parker and Nicky Loomis, Los Angeles Times

LOS ANGELES — Robin Williams, a comic and sitcom star in the 1970s who became an Oscar-winning dramatic actor, died Monday at 63 in Marin County. The Marin County Sheriff’s Office said he appears to have committed suicide.

The news of the beloved actor’s death rocked the nation. Channels broke into their usual programming to make the announcement, and within minutes, he dominated online trending topics.

Williams was hailed as a comic genius and starred in both movies and television for more than three decades. But he also suffered from substance abuse problems.

The actor “has been battling severe depression of late,” his publicist Mara Buxbaum said. “This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time.”

Williams was found unresponsive at his home in the San Francisco Bay Area city of Tiburon at around noon Monday, sheriff’s officials said. He was pronounced dead at the scene.

Dubbed “the funniest man alive” by Entertainment Weekly in 1997, Williams brought audiences hours of laughter, putting his imaginative spin on characters in film and television. He was lauded for his serious roles as well, winning a best supporting actor Oscar for his performance as Sean Maguire, the therapist who counsels Matt Damon’s math genius in Good Will Hunting (1997), and receiving nominations for The Fisher King (1991), Dead Poets Society (1989) and Good Morning, Vietnam (1987).

Williams was known for being open about his problems with cocaine and alcohol over the years.

The actor spent time on a Hazelden campus in Oregon in 2006. He later explained that drinking had gradually become a problem again after 20 years of sobriety.

“You’re standing at a precipice and you look down, there’s a voice and it’s a little quiet voice that goes, ‘Jump,'” the Mrs. Doubtfire star told ABC News in October of that year. “The same voice that goes, ‘Just one.’ … And the idea of just one for someone who has no tolerance for it, that’s not the possibility.”

This summer, he returned to rehab to “fine-tune” his sobriety.

Born in Chicago in 1951, Williams became one of only two students accepted into John Houseman’s prestigious acting program at Juilliard, the other being Christopher Reeve, who became a lifelong friend.

Williams gained fame as Mork, the bizarre, suspenders-sporting alien on the sitcom Mork & Mindy, a spinoff from Happy Days. Williams departed from the script so often that producers intentionally left blank moments on page for him to have space to indulge his ad-libbing genius.

That talent for ad-libbing functioned as a gift and a shield.

“He was always in character — you never saw the real Robin,” said Jamie Masada, founder and chief executive of the Laugh Factory. “I knew him 35 years, and I never knew him.”

“He was a wonderful guy,” Masada added. “I remember John (Belushi) and Robin, both of them always complained to me — no matter where they were people would recognize them. They sold their privacy to the public. They could be in the middle of talking in the street and someone would come up for an autograph. … He (Robin) didn’t realize how much he sold his privacy to people.”

Williams’ last post on Twitter was about his daughter.

Other celebrities turned to the social media site to mourn him.

“I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul,” fellow actor-comedian Steve Martin said on Twitter.

“Robin Williams was like no other,” actor and director Henry Winkler said. “To watch him create on the spot was a privilege to behold. Robin you are an angel now !!! REST IN PEACE”

“Robin Williams, Peace be Still, Peace be Still,” actress Pam Grier posted.

Photo: Ken Hively/Los Angeles Times/MCT