Northern Ireland political parties have suspended their election campaigns as a mark of respect to the victims of the Manchester bomb attack.
Twenty-two people died and scores were injured in the suspected suicide bombing at the Manchester Arena.
Belfast City Council has opened a book of condolence for the victims in the reception of Belfast City Hall.
The taoiseach (Irish prime minister) and the Irish president both sent their sympathies to the people of the UK.
The explosion in Manchester happened at 22:35 BST on Monday at the end of a pop concert by the US singer Ariana Grande.
All events at the SSE Arena in Belfast are currently proceeding as planned, although those due to attend Tuesday’s show with Prof Brian Cox have been advised to leave extra time for “additional security measures”.
SSE Arena officials have held talks with the PSNI and Harbour Police and said they were taking steps to minimise any security risk at the venue.
Greater Manchester Police said officers are treating the Manchester bombing “as a terrorist incident”.
Police believe the attack was carried out by one man, who was carrying an improvised explosive device which he detonated.
Leaders of the Democratic Unionist Party (DUP), Sinn Féin, the Ulster Unionist Party (UUP), the Social Democratic and Labour Party (SDLP) and Alliance have all expressed shock and sympathy to those affected.
DUP leader Arlene Foster said: “Whilst terrorists can bring pain and grief, the kindness and generosity displayed by the people of Manchester has already shown that they will not win.”
Sinn Féin’s Stormont leader Michelle O’Neill said: “For something like that to happen while young people are out enjoying themselves is unthinkable.”
Ulster Unionist leader Robin Swann said the “barbaric attack… was an attempt to destroy our way of life and will not succeed”.
The party has postponed its manifesto launch, which had been due this Thursday, until Thursday of next week.
Suspending the SDLP’s campaign, including its manifesto launch which was due to take place on Tuesday, party leader Colum Eastwood, said: “Today is a day for unity of purpose, not party political difference.”
Alliance leader Naomi Long said: “Those behind these 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.”
Northern Ireland Secretary of State James Brokenshire said it was shocking that so many bright, young lives had been cut short in Manchester.
He tweeted: “Prayers for all those affected by this appalling attack #WeStandTogether”.
A Conservative Party spokesman for Northern Ireland said in a statement: “This is a time for leadership in the UK that stands strongly and unequivocally against terrorism of any kind.”
The Northern Ireland Conservatives, whose manifesto was due to be launched tomorrow have postponed it until a date yet to be decided.
Irish Minister for Foreign Affairs Charlie Flanagan said he was “not at this time aware of any Irish citizens affected” by the bombing.
But he said the Irish government would “stand by our nearest neighbour, the UK, especially due to the strong links between our country and the city of Manchester”.
The outgoing Taoiseach (Irish Prime Minister) Enda Kenny said he was “heartbroken” for all those killed and injured.
“The city of Manchester has exceptionally close ties with our country and I extend the solidarity of the Irish government and all our people to those affected across the UK,” Mr Kenny added.
Irish President Michael D Higgins said: “Manchester has been home to the Irish and so many nationalities for centuries.
“Our thoughts in Ireland are with all of the people of Manchester and our neighbours throughout the United Kingdom at this time.”
Mr Higgins added he was writing a formal letter to Queen Elizabeth to convey the sympathy of the Irish people.
Police in Manchester have set up an emergency telephone number in response to the attack. It is: 0161 856 9400.
The Irish government said any Irish citizens with concerns for family and friends can contact its Department of Foreign Affairs: 00 353 1 408 2000.