Unruh said her son was able to escape when Spacey got up for a moment to use the restroom.
Unruh said she has long wanted to say something about what Spacey allegedly did to her son.
"The victim, my son, was a starstruck, straight 18-year-old young man who had no idea that the famous actor was an alleged sexual predator, or that he was about to become his next victim", Unruh said.
The conference didn't come as a total surprise. "But the climate in this country has changed now thanks to the fearless women and all sexual abuse victims who have come forward with their accuations against Harvey Weinstein and other sexual abusers".
"The #weinsteinscandal has emboldened me- #truth time", she tweeted.
According to the allegations, the assault continued until Mr Spacey went to the toilet and a concerned woman nearby approached to ask if the teenager was okay. "Time the dominoes fell", she tweeted on October 13, in a missive that has accumulated 3,800 retweets and more than 4,700 likes.
"It happened late night inside the Club Car restaurant and bar", she said. "And one way to stop it is for fearless victims and their families to speak out so that children are safe, other victims are empowered and the victims themselves are emboldened". The #headline is #KevinSpacey the #predator- this is not about him being gay'.
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"He stuck his hand inside my son's trousers and grabbed his genitals", Unruh told reporters at a news conference at the Boston Hilton. "When my son was drunk, Spacey made his move".
"Spacey stuck his hand inside my son's trousers and grabbed his genitals", Unruh said.
He said in a statement: 'But if I did behave as he describes, I owe him the sincerest apology for what would have been deeply inappropriate drunken behavior'. Though he was originally too scared and embarrassed to report the incident to police, the now 19-year-old college sophomore has now done so.
Unruh said her son filed a police report last week with the Nantucket Police Department and that a criminal investigation is underway. "He's committed to doing what he can to stop Kevin Spacey from victimizing anyone else".
Attorney Mitchell Garabedian, who sat next to Unruh, said he is conducting his own investigation and did not say whether they intend to file civil action against Spacey.
Spacey has not commented on Unruh's claim.