A silent vigil has been held in Belfast to remember the Manchester bombing victims.
A small group held posters professing "we stand together" outside the gates of City Hall.
Political and church leaders in the North also expressed sympathy.
Amnesty International director in the region Patrick Corrigan said: "Tonight's vigil outside City Hall is really a spontaneous response by members of the local community and really it is an opportunity to stand in solidarity with the citizens of Manchester from the citizens of Belfast.
"To say we share your pain, and it is a response that is about a sense of community, a sense of humanity and the simple message tonight is, we stand together."
Campaigning in the General Election was suspended on Tuesday and the first televised debate involving Stormont's parties, scheduled for Wednesday, has been postponed.
Extra security was introduced for a show by Professor Brian Cox in Belfast's SSE Arena on Tuesday night.
As a book of condolence opened in the City Hall, Democratic Unionist leader and former Stormont first minister Arlene Foster said "terrorism must never win".
She branded the attack "indiscriminate and barbaric" and called for people to unite in condemnation of terror.
"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 Fein's Stormont leader, Michelle O'Neill, was one of the many who signed the book of condolence at City Hall.
She said the bombing was "horrific".
"For something like that to happen while young people are out enjoying themselves is unthinkable," she said.
SDLP leader 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.
"This was a heartbreaking act of barbarism."
Ulster Unionist leader Robin Swann insisted the attackers would not succeed.
"We stand united with the people of Manchester to face down the terrorists who carried out this cowardly act," he said.
Alliance leader Naomi Long said the terrorists would be defeated.
"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," she said.
"That is proof good will triumph over evil."
Church of Ireland Primate of all Ireland Dr Richard Clarke sent a personal message to the Bishop of Manchester, David Walker, conveying his sympathies.
The leader of the Catholic Church in Ireland, Archbishop Eamon Martin, said: "Such an awful attack challenges us all to resolve personally to build peace, solidarity and hope everywhere.
"Only in this way can the hearts of those who plan and perpetrate such violent and pointless attacks be changed."
Moderator of the Presbyterian Church in Ireland Dr Frank Sellar offered his prayers for those affected.
"Wicked deeds deliberately inflicted on innocent people, especially where children and the young are involved, seem to cut the deepest, and I offer our support and heartfelt sympathy to everyone mourning the loss of loved ones and those hoping for good news," he said.