David Cameron and the Queen have extended their sympathies to the families of the 27 murdered children and adults who were victims of one of the worst mass school shootings in American history.
In a message sent to Barack Obama on Friday evening, the monarch said she was "deeply shocked and saddened" to hear of the shootings.
The message, which began "To President Obama", read: "I have been deeply shocked and saddened to learn of the dreadful loss of life today in Newtown, Connecticut; particularly the news that so many of the dead are children.
"Prince Philip joins me in extending our heartfelt sympathy to you and the American people at this difficult time.
"The thoughts and prayers of everyone in the United Kingdom and throughout the Commonwealth are with the families and friends of those killed and with all those who have been affected by today's events."
The message was signed Elizabeth R.
The prime minister took to Twitter to say he was "devastated" by the Connecticut shootings and described the death of so many children as "truly heartbreaking".
Cameron later issued a statement, saying: "I was shocked and deeply saddened to hear about today's horrific shooting at Sandy Hook Elementary School in Connecticut that has killed and injured so many innocent people," he said.
"My thoughts are with the injured and those who have lost loved ones.
"It is heartbreaking to think of those who have had their children robbed from them at such a young age, when they had so much life ahead of them.
"I offer my sincere condolences to the families, to president Obama, governor (Dannel) Malloy and the American people. The thoughts of the British people are with you all at this very difficult time."
In an emotional televised address from the White House, he said: "I offer governor Malloy my condolences on behalf of the nation, and made it clear he will have every single resource that he needs to investigate this heinous crime, care for the victims, counsel their families.
"We endured too many of these tragedies in the past few years. And each time I learn the news, I react not as a president, but as anybody else would - as a parent. And that was especially true today. I know there is not a parent in America who doesn't feel the same overwhelming grief that I do.
"The majority of those who died today were children. Beautiful little kids between the ages of five and 10 years old."
The president paused for some seconds and wiped away tears before continuing: "They had their entire lives ahead of them.
Birthdays, graduations, weddings, kids of their own. Among the fallen were also teachers, men and women who devoted their lives to helping our children fulfil their dreams.
"So our hearts are broken today. For the parents, and grandparents, sisters and brothers of these little children, and for the families of the adults who were lost.
"Our hearts are broken for the parents of the survivors as well, for, as blessed as they are to have their children home tonight, they know that their children's innocence has been torn away from them too early, and there are no words that will ease their pain."
Scottish First Minister Alex Salmond said: "I share the shock and deep sadness of people across the world at the terrible loss of innocent lives at Sandy Hook Elementary School in Connecticut.
"It is impossible to comprehend how the children, along with the adult victims, could have been so cruelly taken from their loved ones.
"All of the victims, the grieving relatives and colleagues, and the entire community of Newtown are in the thoughts and prayers of people across Scotland tonight. I send them my deepest condolences."
Liberal Democrat president Tim Farron tweeted: "The senseless murder of innocent children in Connecticut shocks us all. My thoughts & prayers are with the families affected tonight."