Barack Obama will travel on Sunday to Newtown, Connecticut, the site of the deadly elementary school shooting.
He is expected to meet with families of the victims and will speak at a service.
The National Football League has asked all home teams to observe a moment silence for the victims of the Connecticut school shooting prior to the opening kick off this weekend, according to the NFL website.
The New York Giants will wear a decal on their helmets in memory of the victims, wearing the initials S.H.E.S. (Sandy Hook Elementary School) on the back of their helmets.
Russian leader Vladmir Putin has called the shootings in a school in Connecticut "particularly tragic" as the majority of the 28 killed were children.
The Kremlin said in a statement:
German Chancellor Angela Merkel has said her "deepest sympathy" is reserved for relatives of the victims of the shooting rampage that left 28 people dead, including 20 children in Connecticut. In a statement, the German Chancellor said:
President Barack Obama said the nation was grieving for those killed during the "tragic shooting" in Newtown, Connecticut.
During his weekly radio and television address, the President said, "We have endured far too many of these tragedies in the last few years ... We have to come together and take meaningful action to prevent more tragedies like this. Regardless of the politics".
New York Governor Andrew Cuomo, New Jersey Governor Chris Christie and Connecticut Governor Dan Malloy tweeted:
The principal of Columbine High School has offered his condolences to the families of the victims killed during a school shooting in Newtown, Connecticut.
"It's a reminder of how vulnerable we are as a society," he said.
"As a society we have to come together, we have to stop these senseless deaths." reports the Denver Post.
The Brooklyn Nets basketball team held a moment of silence for victims of the Newtown shooting ahead of their game with the Detroit Pistons at the Barclays Center in Brooklyn, New York.