A man who claims that Kevin Spacey sexually assaulted him said the Hollywood actor was an aggressive "predator" uncomfortable with his sexuality, a London court was told on Monday.

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

The alleged victim described Spacey, the former artistic director of London's Old Vic theatre, 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, who won Oscars for "The Usual Suspects" and "American Beauty", 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.

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

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. (AFP)