The film star and UN Ambassador for Women said the website targeted her after she spoke up for women’s rights.
She said: “After I gave my speech [at the UN] there was a website threatening to release naked pictures of me.
“I knew it was a hoax, I knew the pictures didn’t exist, but I think a lot of people that were close to me knew gender equality was an issue but didn’t think it was that urgent, that it was a thing of the past.
“And then when they saw that the minute I stood up talking about women’s rights I was immediately threatened, I think they were really shocked, my brother was particularly upset.
“This is a real thing that’s happening now, women are receiving threats.
“I was raging, it made me so angry, I was like, this is why I have to be doing this. If anything, if they were trying to put me off it, it did the opposite.”
Last September a web page entitled Emma You Are Next, featuring an image of the Harry Potter star next to a countdown clock, appeared to have been created by a user of image-sharing website 4chan, on which lewd photos of celebrities including actress Jennifer Lawrence and model Kate Upton were posted last month.
Watson was speaking at Facebook’s headquarters in London to mark International Women’s Day.
She said her most positive role model was her mother, who taught her that her deeds were more important than her looks.
She said: “The obvious choice is my mum, she was a single mother and a type one diabetic, so to see her strength and resilience was really inspiring growing up.
“I think she instilled in me in my teenage years when I was feeling very insecure, that what I was thinking, doing, saying, were infinitely more important than my physical appearance.
“She really encouraged me to be an individual. I remember her being thrilled when I got my first detention because she was really worried I was going to be a bit straight laced.
“I’d failed my Latin exam ... I think I’d failed it a few times actually, maybe more than once, but let’s not go into that.”
Speaking in support of the He For She campaign, she added: “Our society in general devalues the she, I mean the qualities that are associated with the feminine that are found in all of us.
“There’s this imbalance, this distortion that is just hindering our progress, it’s causing discord, violence and fear the world over.”
“It’s uncomfortable, it’s awkward to acknowledge there is a problem, but we need to understand we are complicit.
“Another anecdote is that a lot of the criticism I have had in my life, some of the harshest moments have been comments from other women.
“It’s not enough to ask men to come and support us.
“We really need to support each other, we really do. So, I guess I would say be brave enough to acknowledge that things are not there yet and support each other.”
One audience member asked Watson if she was paid less than her male counterparts in the Harry Potter films.
She replied: “I wouldn’t dream of complaining about my financial circumstances, but yes, it is a big problem in my industry, it is a huge problem that needs to be addressed.”
She expressed her support for LGBT communities and trans people, saying “this is their movement”.
She said: “He For She is about men coming to support women for femininity and for feminine qualities, because they are currently valued less in our society.
“Femininity needs to be embraced wherever it is ... whether it be in a man or a woman or a non-conforming gender person.
“My specific mandate is to advocate for women and girls but I also understand that these oppressions are interlocking, mutually reinforcing. Inter-sectionality is a really important word here.
“We need to be supporting each other, I hope they feel this is their movement, because it is.”
She said that on the issue of equal pay, society was “straggling behind, it’s just sort of stagnating at the moment”.
Another audience member asked what Watson would say to people who claim women should be paid less because they are more likely to take maternity leave.
She said: “I have come across this argument. I guess I would just say that maternity leave in this country is around two months and I don’t think in the grand scheme of things that is going to hinder a women from achieving, or being incredibly effective, in whatever her field is.”