Natalie Portman was 12 in her first film. The next year, a man sent her his 'rape fantasy'.

Natalie Portman turned 12 on the set of her very first film.

That film was The Professional, and Portman played a young girl who forged a bond with a hit man in the wake of her drug-dealing father and rest of her family being killed by crooked cops.

As Portman’s character navigated her youth and came to terms with her “womanhood”, “desire” and “voice”, Portman too found herself going through the motions.

Who was she? Who did she want to be?

It turns out, she didn’t have much time to think. The world was very quickly deciding for her.

Facing thousands of defiant women at Saturday’s Women’s March, Portman told the story of how, when her first fan letter arrived, she was desperately excited to open it. She was getting noticed, and as a result, so too was her work. Her art.

It was a rape fantasy a man had written her.

It was the beginning, she says, of consistent, tiny realisations that the world wasn't good at handling young women.

"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 told the crowd. "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."

Portman said she expressed herself as "prudish, conservative, nerdy and serious" as a means of being heard and feeling safe.

"At 13 years old, the message from our culture was clear to me," she said. "I felt the need to cover my body and to inhibit my expression and my work in order to send my own message to the world that I'm someone worthy of safety and respect.

"The response to my expression, from small comments about my body to more threatening deliberate statements, served to control my behaviour through an environment of sexual terrorism."

It's something, standing on stage in front of thousands of resistant women, she said she's fighting and fighting and fighting to change.