Letters of feuding being thrown around Hollywood! It could get nasty...

Sharon Osbourne has officially joined her daughter's feud with Lady Gaga, sending her and old fashion letter calling her a hypocritical jerk who encourages her fans to harass her daughter Kelly.

Osbourne is respondingto a letter Gaga wrote to Kelly Osbourne and posted on her official website in which Lady Gaga blasted Kelly as a "bully" who unfairly picks on celebrities on her TV show "Fashion Police."

Gaga said she's disgusted that Kelly and Joan Rivers constantly make "jokes about artists and celebrities as if we are zoo animals."

Sharon says Gaga's fans have been waging a campaign of online terror against the Osbournes thanks to Gaga's criticism of the show. Sharon claims some of the singer's fans have even made death threats to Kelly.

In her letter to Gaga, Sharon demands Gaga instruct her fans to stop the harassment ASAP.

More to come on her response.