(CNN) Natalie Portman said experiencing "sexual terrorism" at the age of 13 made her feel the need to cover her body and inhibit expression.

The award-winning actress shared her traumatic experience Saturday while addressing a crowd of thousands gathered in downtown Los Angeles for the Women's March

Portman remembered turning 12 on the set of "Léon: The Professional." It was her first film. She played a young girl who befriended a hit man in hopes of avenging the murder of her parents, she said.

A year later, when the movie was released, she opened her first fan letter. It was a "rape fantasy" from a man.

"A countdown was started on my local radio show to my 18th birthday -- euphemistically the date that I would be legal to sleep with," she said. "Movie reviewers talked about my budding breasts in reviews. I understood very quickly, even as a 13-year-old, that if I were to express myself sexually I would feel unsafe and that men would feel entitled to discuss and objectify my body to my great discomfort."

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