The actresses took a moment to sign an e-card to pledge support to the Sandy Hook Promise group, which supports families affected by the elementary school shooting in December (12) which left 20 children and six adults dead.
In a post on her Twitter.com page, Moore writes, "Just signed sandy hook's card supporting Moms of Gun Violence victims on Mothers Day. You can too", while Hart posted a similar message.
Chris Brown, Sean 'Diddy' Combs, Ricky Martin and Kelly Osbourne were also among the celebrities who took to their Twitter accounts to mark the special day, with the singer-turned-reality Tv star praising her mother Sharon.
She wrote, "To cherish, love all. realize my dreams & always appreciating my many blessings I learned all of these things from you MrsSOsbourne I Love U", while Brown added, "I wanna praise the woman who created me! 'Momma Joyce' happy Mother's Day. Thank u for always letting me be creative and dream! Love you mom."
Cher posted, "I want to send my love, to mothers everywhere! I know what a hard job it is, because I've fallen short So Many times!", Combs declared, "Happy Mother's Day to all of the beautiful mothers out there", and Paula Abdul shared, "No matter what you give Your Mom... presents aren't nearly as important as Presence. Time spent is priceless."
Martin, who is father to twin boys born via a surrogate, writes, "Happy Mothers Day! Specially mine (Nereida)... And also to all those amazing surrogate mothers out there! You are a blessing!" and Missy Elliott gushed, "Not just 2day but every day is Happy Mother's Day to all the Great moms!"
New mother Beverley Mitchell revealed her first Mother's Day since giving birth to daughter Kenzie was set to prove challenging, stating, "Kenzie is certainly having a day! Making me work hard to earn my first Mother's Day. Thank goodness Michael & I are taking turns. meltdown."