Charlotte Lewis, 42, said Friday that the filmmaker abused her "in the worst possible way" sometime in the 1980s.
Lewis provided no evidence to support her claims, and her attorney, Gloria Allred, did not permit her to answer questions during a news conference in her office.
However, Allred said the woman provided evidence to a police detective and officials from the Los Angeles County district attorney's office. She refused to provide specifics and also refused to answer questions about whether her client's allegations involved drugs or rape.
"Our detectives did conduct the interview but the department has not begun an investigation," said police spokesman Richard French. He did not know when the interview was conducted.
It was unclear what year the alleged assault took place.
"He took advantage of me and I have lived with the effects of his behavior ever since it occurred," said Lewis, reading from a prepared statement at a news conference in her lawyer's office. "All I want is justice."
Allred said that, to her knowledge, no criminal complaint or lawsuit was made in France over the alleged assault.
The duty officer for the French Justice Ministry said late Friday night that he was not aware whether the British actress had filed a complaint in France about her allegation.
A statement released by Polanski's U.S. legal team said his attorneys had no information about the allegations that were made at the press conference "but we do know that our district attorney continues to refuse to provide the Swiss government with accurate and complete information relevant to the extradition issue."
Polanski, the Oscar-winning director of "Rosemary's Baby," "Chinatown" and "The Pianist," is under house arrest at his Swiss chateau in connection with a 33-year-old case.
Polanski was accused of drugging a 13-year-old girl with champagne and part of Quaalude and raping her at Jack Nicholson's house in Los Angeles in 1977. He pleaded guilty to unlawful sexual intercourse but fled to France before sentencing and remained a fugitive until Swiss authorities arrested him on Sept. 26 on a U.S. warrant as he arrived in Zurich to receive a lifetime achievement award from a film festival.
Lewis said she came forward because she heard that Polanski was fighting extradition to the United States and "that his legal team is portraying his previous offense against a minor as an isolated instance."
Allred said she doesn't plan any legal action right now, such as a lawsuit, but believes the allegations would be relevant when Polanski is sentenced.
Her client is ready to testify if necessary, she said.
"If the judge believes these claims, it could certainly have an impact on the court's decision," she said.
Lewis played a role in Polanski's 1986 movie, "Pirates." She's also credited with a role in the 1986 movie "The Golden Child" and small roles in other movies and TV shows.