London UK - A man who claims that Kevin Spacey sexually assaulted him said the disgraced Hollywood star was an aggressive "predator" uncomfortable with his sexuality.

The alleged victim, who cannot be identified for legal reasons, claims that the Oscar-winning star began sexually assaulting him more than two decades ago.

But despite him making clear to Spacey that his advances were unwanted, the actor persisted, a jury at Southwark Crown Court in south London heard.

Spacey has pleaded not guilty to 12 sexual offenses, including indecent assault, against four men between 2001 and 2013.

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The alleged victim described Spacey, the former artistic director of London's Old Vic theater, as a "slippery, snaky, difficult person."

Good-looking young men were warned to be on their guard because it was "well known that he was up to no good," he said, likening the actor to his serial killer character in the film Se7en.

"He's a bit like that, a bit creepy," he said.

The prosecution has described Spacey as a "sexual bully," whose preferred method of assault was grabbing other men in the crotch.

His first alleged victim recounted in a videotaped police interview played to the jury that the actor grabbed him on several occasions, and was told to stop.

He told officers that Spacey once grabbed him painfully in a car. He pushed back and told the star: "Don't do that again or I will knock you out."

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The actor allegedly replied: "That's such a turn-on to me, you're such a man."

The witness said Spacey's unwanted sexual overtures made him uncomfortable, ashamed, and disgusted, so much so that he could not watch any drama with the actor in it.

"I can't stand watching the man. It makes me feel sick," he told police, describing Spacey as a "predator" who was "aggressive."

"He was obviously very messed up with his sexuality," he added.

Under cross-examination by Spacey's lawyer Patrick Gibbs, the alleged victim denied "100, one million percent" that he had any feelings for the actor, or that what happened led him to question his own sexuality.

He reported the claims to the police after publicity about other allegations against Spacey, he said.

His only motivation was "to tell the truth," he added.