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LOS ANGELES, March 4 (Reuters) - Allison Janney won her first Oscar on Sunday for her supporting role as the demanding and abusive mother to Olympic figure skater Tonya Harding in the mockumentary "I, Tonya."

Janney, 58, a seven-time Emmy winner, swept all major acting honors this season, including Screen Actors Guild, Golden Globe and BAFTA awards.

"I did it all by myself," Janney said facetiously to laughter from the audience. "Nothing further from the truth."

Janney plays LaVona Golden, the mother of U.S. Olympic figure skater Tonya Harding, in the independent comedy about the disgraced and troubled athlete who is best remembered for her role in a 1994 assault on rival skater Nancy Kerrigan.

Janney is most familiar to audiences from her roles on 2000's White House television drama "The West Wing" and current TV comedy "Mom." Her past film credits include supporting roles "America Beauty," "The Hours" and "Juno."

"I, Tonya" screenwriter Steven Rogers wrote the role of the hard-driving, foul-mouthed LaVona expressly for Janney, partly as a way for her to demonstrate her range.

"Steven Rogers, look what you did, look at what you did," Janney said accepting the award. "You're a brilliant writer. Thank you for the gift of LaVona. I did not see this coming. You did. You give new meaning to the word friend."

The actress is almost unrecognizable in the movie behind a wig, large glasses and with a parakeet on her shoulder.

"He wanted to show people what else I could do, that I could play someone this far away from myself and yet make her real and grounded," Janney said last year. "I have to say it's hard to say you had fun playing someone so awful, but it was a fun challenge."

Janney's first passion was ice skating and she had hopes as a teenager of competing in the Olympics before an accident and the realization that she was too tall to succeed ended her dreams.

She later studied acting under Paul Newman at Kenyon College in Gambier, Ohio, and credits Newman's late wife, actress Joanne Woodward with encouraging her career. (Reporting by Eric Kelsey; Editing by Peter Henderson and Sandra Maler)