LONDON—European leaders, echoing people across the continent, expressed horror in offering condolences to the United States after the school massacre in Connecticut on Friday.

In an open letter to U.S. President Barack Obama, French President Francois Hollande said he was “horrified.”

“In these tragic circumstances, I want to express my deep shock and sorrow at this act of unspeakable violence at an elementary school that left so many victims,” he said.

British Prime Minister David Cameron said in a statement: “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.”

Though news broke on a Friday evening, limiting official commentary, newspaper websites and broadcasters devoted non-stop live coverage to events unfolding in Newtown, where police said a lone gunman had killed 20 children and six adults.

“Panic! Screaming! Mass operation! Killing Spree in U.S. School,” headlined Germany’s top selling Bild newspaper.

Germany, France, Britain and other European countries have suffered similar mass shootings—in Norway, one man killed 77 people only last year—but commentators in Europe were quick to point to Americans’ much higher levels of personal gun ownership as a factor in the frequency of shootings there.

“If not now, when is the time to talk about gun control?” ran the headline on an article in Britain’s Guardian newspaper.