Theresa May has said the “callous terrorist attack” in Manchester, which left 22 dead, was one of the worst ever experienced in British history.
The prime minister said the bombing was an act of “sickening cowardice” with young people having been deliberately and indiscriminately targeted.
She said the country’s spirit would not be “broken” and it must “take on and defeat” the ideology behind the attack.
General election campaigning has been suspended until further notice.
Saying her thoughts and prayers were with the families of those killed and the injured, the PM said there would be difficult days ahead but Britain’s “spirit will never be broken… terrorists will never win and our way of life will always prevail”.
- 22 dead and scores hurt in Manchester blast
- Manchester explosion: Latest updates
- SNP cancels manifesto launch after blast
- NI campaigns suspended after attack
Speaking in Downing Street, she said it was “beyond doubt” that the attack at the Ariana Grande concert, in which children were among the dead and more than 50 people were injured, was “among the worst terrorist incidents we have ever experienced in the UK”.
“All acts of terrorism are cowardly attacks against innocent people but this attack stands out for its appalling, sickening cowardice, deliberately targeting innocent and defenceless young people,” she said.
Mrs May said the police were working to establish the full facts and although they believed they knew the attacker’s identity, this would not be confirmed while they investigated whether he acted alone or had been helped by others.
The prime minister is set to travel to Manchester this afternoon before leading another emergency response meeting of ministers, police officers and security officials – known as a Cobra meeting named after Cabinet Office room in which they meet – later on Tuesday.
Labour leader Jeremy Corbyn, who spoke to Mrs May overnight, said he “totally and unreservedly condemned this appalling act of violence”.
Praising the emergency services who were helping people during this “terrible and traumatic time”, he said the country must come together and “not allow this violence to divide us or prevent us from leading a decent and normal life”.
SNP leader Nicola Sturgeon said the news was “heartbreaking” and her thoughts were with the victims of the “barbaric” attack.
Announcing it would not be going ahead with its manifesto launch as planned, the party tweeted: “Our thoughts are with Manchester.”
The Lib Dems, UKIP, Green Party and Plaid Cymru have also announced that they will be ceasing campaigning until further notice.
Lib Dem leader Tim Farron, who cancelled a planned visit to Gibraltar, praised the “great bravery and heroism” of the emergency services
“This is an attack on innocent people and the nation is united both in its grief and its determination to stand up to this deplorable attack,” he said.
UKIP leader Paul Nuttall said his thoughts and prayers were with those affected while Green Party co-leader Jonathan Bartley said “for young people to be targeted in this way is utterly atrocious”.
Tuesday’s Cobra meeting was attended by senior ministers, including Foreign Secretary Boris Johnson, police officers and intelligence officials.
Speaking before the meeting, Home Secretary Amber Rudd said the attack had “deliberately targeted some of the most vulnerable in our country”. She praised the emergency services for their response to the incident.
Union flags are flying at half mast in Downing Street as a mark of respect while the BBC announced that a series of Andrew Neil TV interviews with party leaders would be halted while campaigning was suspended.
Foreign leaders have also expressed their condolences and solidarity with the UK in the face of the global terror threat.
US President Donald Trump, who is on a trip to the Middle East, said the “slaughter of innocent people, mostly innocent children” was wicked and the work of an “evil loser”.
He called on all “civilised nations to join together to protect human life” and rid violent extremism and terrorism “from our society for ever”.
French Prime Minister Edouard Philippe said the attack was “abominable”.
“The most cowardly form of terrorism has struck once more, targeting, as it did in Paris more than a year ago, a concert venue, aimed specifically and knowingly at very young girls gathered together for a moment of celebration and joy,” he said.
Jean-Claude Juncker, the president of the European Commission, said those wanting to “instil fear and sow division” would not prevail.
“Today we mourn with you,” he said. “Tomorrow we will work side by side with you to fight back against those who seek to destroy our way of life. They underestimate our and your resilience.”
Australian Prime Minister Malcolm Turnbull said his country stood with the UK “as we always have and always will, steadfast allies in freedom’s cause”.