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.
US President Donald Trump on Tuesday condemned the "evil losers" behind a bomb attack at a pop concert in the British city of Manchester that killed at least 22 people.
"So many young, beautiful, innocent people living and enjoying their lives murdered by evil losers," Trump said after meeting Palestinian president Mahmud Abbas in the occupied West Bank.
"I won't call them monsters because they would like that term. They would think that's a great name."
"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."
I am horrified by the horrendous events in Manchester. My thoughts are with families and friends of those who have died and been injured. pic.twitter.com/QoCXH8qPYW
— Jeremy Corbyn (@jeremycorbyn) May 23, 2017
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."
My thoughts with the victims and their families, and the people of Manchester. Want to thank and pay tribute to emerg services helping today
— Philip Hammond (@PhilipHammondUK) May 23, 2017
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."
Manchester terror attack: What we know
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."
Our heartfelt sympathy and resolute solidarity is with the people of the United Kingdom. Full statement here: https://t.co/FwtDITvVU9 pic.twitter.com/KYo72xl7P7
— Malcolm Turnbull (@TurnbullMalcolm) May 23, 2017
Islamic State supporters celebrate Manchester attack online
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."
Condeno el ataque de Manchester. Mi pesar a las familias de las víctimas fallecidas y mis deseos de pronta recuperación a los heridos. MR
— Mariano Rajoy Brey (@marianorajoy) May 23, 2017
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."
Italia si unisce al popolo e al governo britannici. I nostri pensieri alle vittime dell'attacco di #Manchester e alle loro famiglie
— Paolo Gentiloni (@PaoloGentiloni) May 23, 2017
Prime Minister Lars Lokke Rasmussen said: "Horrendous loss of innocent lives in #Manchester. Our thoughts and sympathy are with the British people."
Horrendous loss of innocent lives in #Manchester. Our thoughts and sympathy are with the British people.
— Lars Løkke Rasmussen (@larsloekke) May 23, 2017
New York steps up security after Manchester concert blast
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."
Canadians are shocked by the news of the horrific attack in Manchester tonight. Please keep the victims & their families in your thoughts.
— Justin Trudeau (@JustinTrudeau) May 23, 2017
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."
https://twitter.com/tsipras_eu/status/866919341702479872
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.
[fbpost link="https://www.facebook.com/bundeskanzler.christian.kern/posts/807967046025273"]
Foreign Minister Sebastian Kurz tweeted that his "thoughts are with the victims' families and friends".
Horrible attack in #Manchester proofs importance of common @OSCE action in countering terrorism. #UnitedCVE @AUT_OSCE
— Sebastian Kurz (@sebastiankurz) May 23, 2017
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