By Ann Saphir, Alex Dobuzinskis and Piya Sinha-Roy

SAN RAFAEL Calif./LOS ANGELES (Reuters) - Oscar-winning actor and groundbreaking comedian Robin Williams hanged himself with a belt in his Northern California home after he had sought treatment for depression, a coroner said on Tuesday based on preliminary findings.

Williams, 63, was found dead by his personal assistant at midday on Monday in a bedroom. He was suspended from a belt wedged between a closet door and a door frame, in a seated position just off the ground, Marin County's assistant chief deputy coroner, Keith Boyd, told a news conference.

"Mr. Williams' personal assistant became concerned at approximately 11:45 a.m. when he failed to respond to knocks on his bedroom door," said Boyd.

"His right shoulder area was touching the door with his body perpendicular to the door and slightly suspended. Mr. Williams at that time was cool to the touch with rigor mortis present in his body," Boyd added.

The official preliminary cause of death was asphyxia due to hanging, he said, and conclusion of the investigation is still weeks away.

Officials also found a pocket knife near Williams and superficial cuts on his left wrist with dried red material that matched what was on the knife blade. It was not yet known if it was his blood.

Williams had been open about his struggles with alcohol and cocaine and in the past months had entered a rehabilitation centre to help him maintain sobriety. But many questions remained over what could have led him to take his own life.

Williams' publicist, Mara Buxbaum, said on Monday that he had been suffering from severe depression, and Boyd acknowledged that he had been seeking treatment without giving more details.

His tragic end stood in stark contrast to the many on-screen characters he portrayed who encouraged those around them to tap into their own inner vitality, a wellspring of creativity to which he himself gave full vent in films such as "Good Morning, Vietnam" and "Dead Poets Society."

Williams was last seen alive by his wife, Susan Schneider, on Sunday night when she retired for the evening. She left the next morning around 10 a.m., thinking that her husband was still asleep.

Boyd would not say whether Williams had left a suicide note, nor if any drugs or alcohol were involved. The full toxicology report would take two to six weeks, he said.

In addition to his wife, Williams is survived by three grown children - daughter Zelda, and sons Cody and Zachary. Funeral arrangements are pending and his body has been released by the coroner facility in neighbouring Napa County.

'THANKS CHIEF'

Tributes poured out from actors, comedians, politicians and generations of fans, including President Barack Obama who called him a "one-of-a-kind" actor.

A force of manic energy, Williams long ago established himself as one of the world's most beloved comedians, and took audiences on wild flights of imagination that often stressed one simple message: seize the day.

His improvisational stand-up routine broke all rules, whether he was giving a comedic account of a nuclear accident in the style of Shakespeare or grabbing a camera from an audience member and pointing the lens down his pants.

Ben Affleck, whose breakthrough role came alongside Williams and Matt Damon in 1997's "Good Will Hunting," for which Williams won his only Oscar, said he was heartbroken.

"Thanks chief - for your friendship and for what you gave the world," Affleck wrote on his Facebook page. "Robin had a tonne of love in him. He personally did so much for so many people. He made Matt and my dreams come true. What do you owe a guy who does that? Everything."

Spontaneous acts of tribute sprang up at landmarks from his career.

In Boston, scores of people jotted tributes in chalk to Williams near at bench in the lush Public Garden downtown, which featured in "Good Will Hunting".

Mourners hung signs including "You will be missed" and "RIP Robin" on the wooden fence of the home in Boulder, Colorado, where parts of the intro credits for his breakout 1970s TV comedy, "Mork & Mindy," were filmed.

On the Hollywood Walk of Fame, fans congregated around Williams' star, leaving flowers and candles to honour the actor.

"My kids grew up on 'Mrs Doubtfire'," said Erlinda Fantauzzi, referring to the hit movie in which he played a father who took on the persona of a tender British nanny to be close to his kids. "I feel so bad. He was a tortured soul and he died alone. He touched adults and children," she said.

Interest in his film work spiked on Tuesday, with "Dead Poets Society," "Mrs. Doubtfire," and "Good Morning, Vietnam" making it into the top 20 in the iTunes movie chart.