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 had been compromised to access the images.
Spider-Man actress Kirsten Dunst, who is also reportedly a victim of the leak, tweeted: "Thank you iCloud."
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".
Other stars said to have been affected include Avril Lavigne, Cat Deeley and Rihanna, with actress Mary Elizabeth Winstead, whose movies include A Good Day To Die Hard, already acknowledging pictures in which she is featured are genuine.
She wrote online: "To those of you looking at photos I took with my husband years ago in the privacy of our home, hope you feel great about yourselves."
Brown Findlay, best known for her portrayal of Lady Sybil in Downton, is said to feature in a video in which she is naked and appears alongside an unnamed man.
A piece of computer code that repeatedly guesses passwords has been found online. The script was posted to software site GitHub, but a message has since appeared saying that Apple had fixed the bug.
According to the post, the script uses the top 500 most common passwords approved by Apple to try to gain access to user accounts. If successful, it would give the hacker full access to the iCloud account, including photographs.
iCloud is Apple's own cloud service, a wireless storage facility that can be used to access files remotely.