Campaigning ahead of next month's UK general election was suspended in the wake of a deadly suspected terror attack at a concert in Manchester.
Twenty-two people have been killed, and 59 wounded, after the explosion at the end of a concert by US singer Ariana Grande at the Manchester Arena.
Labour leader Jeremy Corbyn said he had spoken to Prime Minister Theresa May and they had agreed that all national campaigning would be suspended until further notice.
Mrs May chaired a meeting of the government's emergency Cobra committee this morning.
Liberal Democrat leader Tim Farron called off an election visit to Gibraltar and the SNP postponed today's planned manifesto launch.
The five main political parties in Northern Ireland also suspended campaigning for the day.
Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services.
— Jeremy Corbyn (@jeremycorbyn) May 22, 2017
Mr Corbyn said: "I am horrified by the horrendous events in Manchester last night. My thoughts are with families and friends of those who have died and been injured.
"Today the whole country will grieve for the people who have lost their lives.
"I have spoken with Andy Burnham, the mayor of Manchester, who has fully briefed me on the operational response in the city.
"I would like to pay tribute to the emergency services for their bravery and professionalism in dealing with last night's appalling events.
"I have spoken with the Prime Minister and we have agreed that all national campaigning in the general election will be suspended until further notice."
Mrs May said: "We are working to establish the full details of what is being treated by the police as an appalling terrorist attack.
"All our thoughts are with the victims and the families of those who have been affected."
Electioneering was also suspended by all Scottish parties, with First Minister Nicola Sturgeon expected to host a meeting of the Scottish Government's Resilience Committee in response to the explosion.
Ms Sturgeon's Scottish National Party has delayed the launch of its manifesto following the tragedy.
Northern leaders react to tragedy
SDLP chief Colum Eastwood said: "The people of Ireland understand the tragic pain of loss that those in Manchester are feeling.
"We stand with the people of Manchester today, unbroken, unbowed and resolutely determined to defeat those responsible."
He added: "This was a heartbreaking act of barbarism."
Ulster Unionist leader Robin Swann said the attackers would not succeed.
"We stand united with the people of Manchester to face down the terrorists who carried out this cowardly act.
"As a parent, I can only imagine the hurt and pain which the parents of the children killed and injured are going through."
Democratic Unionist leader Arlene Foster tweeted her condolences.
I have asked all DUP candidates to suspend their campaigning today. 22 dead including children. Horrific injuries. Terrorism must never win.
— Arlene Foster (@DUPleader) May 23, 2017
She said: "Terrorism must never win."
Sinn Féin's leader in the north, Michelle O'Neill, said the attack was horrific.
"For something like that to happen while young people are out enjoying themselves is unthinkable."
Alliance leader Naomi Long said those behind the attacks cannot be allowed to win.
"They try to threaten our very way of life but, in doing so, inspire people to show the best of humanity by presenting generosity and compassion.
"That is proof good will triumph over evil."
