Update 12.25pm: The Queen of England has expressed her "deepest sympathy" to all those affected by the Manchester bomb attack, adding that "the whole nation has been shocked by the death and injury".
Her statement read: "The whole nation has been shocked by the death and injury in Manchester last night of so many people, adults and children, who had just been enjoying a concert.
"I know I speak for everyone in expressing my deepest sympathy to all who have been affected by this dreadful event and especially to the families and friends of those who have died or were injured.
"I want to thank all the members of the emergency services, who have responded with such professionalism and care.
"And I would like to express my admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity."
Update 9.40am: Russian President Vladimir Putin has condemned the blast as a "cynical, inhuman crime", while German Chancellor Angela Merkel called the attack "incomprehensible".
French president Emmanuel Macron said his country is at Britain's "side in mourning", and Australia's prime minister Malcolm Turnbull called it an "attack on innocents".
Meanwhile US First Lady Melania Trump has tweeted her "thoughts and prayers" to the affected families.
My thoughts and prayers to the families of #Manchester.— Melania Trump (@FLOTUS) May 23, 2017
Update 9:30am: President Donald Trump has expressed his "deepest condolences" to the victims of the Manchester bombing calling attackers "evil losers".
Earlier: Leaders from around the world have condemned the terror attack at a Manchester music concert and offered their condolences to the victims.
Australian Prime Minister Malcom Turnbull told the country's parliament the incident was a "brutal attack on young people everywhere".
He said: "This incident, this attack, is especially vile, especially criminal, especially horrific because it appears to have been deliberately directed at teenagers.
"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."
Indian Prime Minister Narendra Modi tweeted: "Pained by the attack in Manchester. We strongly condemn it. Our thoughts are with the families of the deceased & prayers with the injured."
In Japan a government spokesman said: "If this is a terrorist attack, such abhorrent acts of terrorism cannot be justified for any reason, and Japan firmly condemns such an act of terrorism.
"I would like to express my heartfelt condolences to the victims and families of the deceased and my prayers to a swift recovery for the wounded. Japan stands in solidarity with the people of the UK."
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."
President of the European Council, Donald Tusk, tweeted: "My heart is in Manchester this night. Our thoughts are with the victims."
White House press secretary Sean Spicer said President Donald Trump, who is currently on a visit to Israel, is being updated on the situation.
Mr Spicer tweeted: "National Security Team is providing updates to @POTUS on situation in #manchesterexplosion".
French president Emmanuel Macron will speak to Mrs May about the attack later.
In a statement, his office said: "It's with dismay that Mr Macron learned of the murderous attack which struck last night during a concert at the Manchester Arena.
"He expresses to the British people all the compassion and care of France which is at their side in mourning, with a particular thought for the victims and their families."