Kevin Spacey is an aggressive "sexual bully" who assaults men and does not respect personal boundaries or space, a British prosecutor said on Friday at the start of the Oscar-winning U.S. actor's trial on sex offence charges.

Spacey, 63, has pleaded not guilty to a dozen allegations of historic sex offences committed against four men, then aged in their 20s and 30s, which are said to have taken place between 2001 and 2013.

Opening the prosecution case at London's Southwark Crown Court, lawyer Christine Agnew told the jury Spacey was an extremely famous actor who had won a number of awards.

"He is also, the prosecution allege, a man who sexually assaults other men," she said.

"A man who does not respect personal boundaries or space, a man who it would seem delights in making others feel powerless and uncomfortable – a sexual bully. His preferred method of assault is it appears to grab aggressively other men in the crotch."

Spacey, who is being tried under his full name Kevin Spacey Fowler, watched from the dock, wearing a grey suit, a gold-coloured tie and glasses.

The charges against him include repeated incidents of indecent and sexual assaults.

He is also accused of a more serious offence of causing a person to engage in penetrative sexual activity without consent, which carries a maximum punishment of life imprisonment.

"None of the men wanted to be touched by Kevin Spacey Fowler in a sexual way but he doesn't seem to have cared very much for their feelings – he did what he wanted to do – for his own personal sexual gratification," Agnew said.

Describing the allegations of one man, she said Spacey had repeatedly grabbed and groped him, forced his hand onto the actor's genitalia, and had smacked his backside really hard.

"It may well be that the defendant got a sexual thrill out of this type of sexual aggression – it is certainly a consistent feature of three out of the four complainants," Agnew said.

She said the jury would have to decide whether the accusers, some of whom are suing the actor in civil courts, were telling the truth. Spacey's response to the allegations was to say some were made up while other encounters were consensual, Agnew said.

"It is only right that you might feel a little star struck or overwhelmed that you are part of the jury to try such a famous person but ... you must keep yourself grounded and true to the oath that you have taken," Agnew told jurors.

The trial is due to last about four weeks.

Spacey, who won Oscars for best actor in "American Beauty" (1999) and best supporting actor in "The Usual Suspects" (1995), spent more than a decade working in London as artistic director for the British capital's Old Vic theatre from 2004 to 2015.

Once one of Hollywood's biggest stars, he has largely disappeared from public view since being accused of sexual misconduct six years ago.