Prime Minister Theresa May suspended her campaign ahead of a general election on June 8 along with chief opposition leader Jeremy Corbyn.
Here are some of the reactions:
Prime Minister Theresa May called it an "appalling terrorist attack" and suspended her campaign ahead of a general election on June 8 along with chief opposition leader Jeremy Corbyn.
"All our thoughts are with the victims and the families of those who have been affected," said May.
Labour Party chief Jeremy Corbyn tweeted: "Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services."
Finance minister Philip Hammond said: "It is as far as we know a terrorist incident. We're treating it as such. My thoughts are with the victims and families and the people of Manchester and I'd like to give tribute to the emergency services. They've done a great job throughout the night and they're continuing to work at the scene."
German Chancellor Angela Merkel voiced "sorrow and horror", adding: "This suspected terrorist attack will only strengthen our resolve to work with our British friends against those who plan and execute such inhuman acts. I assure the people in Britain: Germany stands by your side."
Russian President Vladimir Putin said he was ready to boost anti-terror cooperation with Britain after "this cynical, inhuman crime.
"We expect that those behind it will not escape the punishment they deserve," he said.
French President Emmanuel Macron voiced "horror and shock" and said he planned to speak to the British prime minister.
Prime Minister Edouard Philippe said it was an example of "the most cowardly terrorism" that was aimed "specifically and knowingly" at young people.
Australian Prime Minister Malcolm Turnbull said the attack was "especially vile, especially criminal, especially horrific because it appears to have been deliberately directed at teenagers."
He added: "This is an attack on innocents. Surely there is no crime more reprehensible than the murder of children. This is a direct and brutal attack on young people everywhere, on freedom everywhere."
Prime Minister Mark Rutte said: "Terrible news from Manchester where a great evening ended in tragedy. Our thoughts are with the victims."
Prime Minister Mariano Rajoy said: "I condemn the Manchester attack. My condolences to the families of the deceased and my fervent wishes that the wounded recover soon."
Italian Prime Minister Paolo Gentiloni tweeted: "Italy joins forces with the British people and government. Our thoughts go out to the victims of the Manchester attack and their families."
Prime Minister Lars Lokke Rasmussen said: "Horrendous loss of innocent lives in #Manchester. Our thoughts and sympathy are with the British people."
Canadian Prime Minister Justin Trudeau tweeted: "Canadians are shocked by the news of the horrific attack in Manchester tonight. Please keep the victims & their families in your thoughts."
Prime Minister Alexis Tsipras said: "We express our solidarity to the British people and our support to the relatives of the victims. We are on your side."
Prime Minister Benjamin Netanyahu said his government "strongly condemns the terrible terror attack.
"Terror is a global threat and the enlightened countries must act together to defeat it in any place," he said.
President Xi Jinping sent his condolences to Queen Elizabeth II, expressing "deep grief for the victims" and their families.
Austrian Chancellor Christian Kern called the Manchester attack "horrible and shocking" in a statement posted on Facebook.
Foreign Minister Sebastian Kurz tweeted that his "thoughts are with the victims' families and friends".