On Friday morning (December 14), suspect Adam Lanza allegedly killed his mother Nancy Lanza then entered Sandy Hook Elementary School, reportedly killing 20 children and six adults.
Lanza took his own life at the school, bringing the death total to 28.
Winfrey posted on her Twitter: "Dear Lord.. Help us to understand as we Stand with these families. Heal our broken hearts. #PrayForNewton."
Bieber expressed: "heard about what happened in CT, my prayers go out to all those suffering in this tragedy. it'ss just wrong. Everyone please pray for them."
Meanwhile, Rihanna posted: "God have mercy!!!! No one deserves this! Praying for the families of the victims of the Connecticut shooting!! What a Christmas it will be."
Other stars who expressed their condolences and shock on Twitter include Anderson Cooper, Larry King, NBA athlete LeBron James, Eva Longoria, Lea Michele, Tyra Banks and Blake Shelton.
Anderson Cooper: "All our words seem so small, so meaningless in the face of such horror."
Larry King: "This is a double horror story-the horror of the senseless deaths & that this continues 2happen in this country."
LeBron James: "WTH!! Once again our country is going through another horrific tragedy. Prayers goes out to all the families in Connecticut. I'm sick!"
Eva Longoria: "My heart sinks as I hear the news of the Sandy Elementary shooting in CT...God Bless everyone and their families!!!"
Lea Michele: "There are no words to express the sadness of this unthinkable tragedy. Sending my prayers to the families in Newton CT."
Tyra Banks: "My deepest condolences to the friends and families of the victims of the CT school shootings."
Blake Shelton: "Stand strong and stand united everyone... We are Americans and we are better than this. What a sad sad disgusting day this is..."
Paramount Pictures also announced that it has delayed the US premiere of Tom Cruise's Jack Reacher due to the Newtown tragedy.
Watch a video of US President Barack Obama's statement on the Newtown school schooting below: