Claire Sinclair

Claire Sinclair Credit: Claire Sinclair (Getty Images)

Hugh Hefner's oldest son was arrested on Sunday after he allegedly attacked his girlfriend, 2011 Playboy Playmate of the Year Claire Sinclair, reported Monday.

Marston Hefner, 21, repeatedly punched Sinclair, kicked her in the stomach and wouldn't let her leave their Pasadena, Calif., home, the 20-year-old Playmate reportedly told cops.

Authorities, alerted by Sinclair's family, found bruises and red marks on her body, reported.

Hefner copped to arguing with his on-again /off-again girlfriend but didn't admit that he hit her, the website said.

The University of San Francisco student was arrested for misdemeanor domestic violence around 11:15 p.m. Pacific time and released on $20,000 bail, according to TMZ.

Yesterday, Sinclair tweeted: "There's two types of pain in the world - pain that hurts you, and pain that makes you stronger. All of your positivity is giving me strength."

She admitted that her relationship with Hefner has had its problems during an interview with Maximo TV last year.

"It's going well right now, but it's very tumultuous, to tell the truth," she said.

Hefner's mom is 1989 Playmate of the Year Kimberley Conrad. 

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