Model Cara Delevingne and former Downton Abbey actress Jessica Brown Findlay are among the stars whose pictures are said to have been posted online.
Watson said: "Even worse than seeing women's privacy violated on social media is reading the accompanying comments that show such a lack of empathy."
American actress Lena Dunham said the person responsible for leaking the pictures was "a sex offender" and anyone looking at them was "violating these women again and again".
Initial reports concluded that Apple's iCloud service, iCloud is Apple's own cloud service, had been compromised to access the images. A spokeswoman for technology giant Apple confirmed it is looking into the issue.
It has been reported that the leak could have been down to an attack on the passwords of the individuals affected.
FBI spokeswoman Laura Eimiller said it was "aware of the allegations concerning computer intrusions and the unlawful release of material involving high-profile individuals, and is addressing the matter", adding that it would be inappropriate to make further comment.
A spokeswoman for Hunger Games actress Jennifer Lawrence, who is one of the biggest stars to be caught up in the scandal, said she had asked US authorities to prosecute whoever is posting the photos, which she said were "a flagrant violation of privacy".
Meanwhile, a New York lawyer who has previously represented Al Pacino, Sean Connery and Robert Redford, said he has advised his clients not to use smartphones and the iCloud.
Martin Garbus said: "Nothing is safe on the internet, period.
"Everything on your iPhone, whether it be phone calls, message texts, pictures, is all available.
"There are just so many different ways that one's privacy can be invaded."