Eight boys and 12 girls, all first grade pupils aged just six or seven, were shot dead at Sandy Hook Elementary School in Newton, Connecticut on Friday.
The shocking tragedy cast its shadow around the world, with millions moved by the extent of the loss and the tales of heroism by the teachers who tried to protect the children in their care.
Six of the school staff - including the head teacher and school psychologist - were also killed.
A book of condolence was opened at Belfast City Hall by DUP Lord Mayor Gavin Robinson, at 3pm on Monday.
"It must be an extremely difficult time for the families and the school community, particularly in the run up to Christmas when families come together," he said, having personally signed the book.
"We want to let them know that we are thinking about them here in Belfast and that they are in our prayers."
I'm deeply saddened by the heartbreaking events at Sandy Hook Elementary School in Connecticut.
Belfast Lord Mayor Gavin Robinson
US Consulate Gregory Burton also joined local people, tourists and even a newlywed couple in leaving written tributes.
One couple visiting from America with their young daughter told UTV they had appreciated the opportunity to express their sympathy to those back home.
"We just want to say, as we're away from home right now, our hearts go out to all the people that have been affected by this tragedy," they said.
"We're just broken for their families. Our prayers and thoughts are with you."
They described waking their little girl on the night that they heard the news, just to give her hugs.
"That's all we could do and we know that we were blessed enough to still be able to do that."
A book of condolence has also been opened in Ballymoney by DUP Mayor Evelyne Robinson.
"I wanted to open a book of condolence for the citizens of Ballymoney Borough to pay their respects to the bereaved families, and to let the local community in Newtown know our hearts go out to them at this time," she said.
"It's so much a time for children with the excitement of Christmas and a time of fond memories as families meet together."
That book will be available at Riada House during office hours until 31 December.
The local sympathies come as the first of the Sandy Hook victims are laid to rest.
Services for six-year-olds Noah Pozner and Jack Pinto are being held on Monday, while Jessica Rekos, also six, will be buried on Tuesday.
On Wednesday, seven-year-old Daniel Barden and teacher Victoria Soto, 27, will be laid to rest.
A service will be held for six-year-old Catherine Hubbard on Thursday.
A date for the private service planned for Dylan Hockley, also six and originally from Hampshire in England, has not been announced.
President Barack Obama told those attending Sunday night's vigil in the grief-stricken town of Newton that he will use "whatever power" he has to in order to prevent a repeat.
The issue of gun control is hugely controversial and tends to be revisited in the wake of school shootings, which have been all too frequent in the US.
"What choice do we have?" Mr Obama said.
"Are we really prepared to say that we're powerless in the face of such carnage, that the politics are too hard? Surely we can do better than this. We have an obligation to try."
Are we prepared to say that such violence visited on our children year after year after year is somehow the price of our freedom?"
President Barack Obama
Adam Lanza, 20, had already shot his mother Nancy Lanza when he went to the school armed with her legally held guns and opened fire, before turning the gun on himself as police approached.
Officers say he had been carrying enough ammunition to kill the whole school if given enough time.
It is understood that Sandy Hook Elementary may never re-open and that education officials are now considering the possibility of sending surviving pupils to a former school in a neighbouring town.
At the vigil for the victims, those in attendance broke down sobbing as Mr Obama slowly read the names of the victims.
"God has called them all home," he said.
"For those of us who remain, let us find the strength to carry on and make our country worthy of their memory."
The president had first met privately with families of the victims and with the emergency personnel who responded to the shootings.
Police and firefighters then received hugs and standing ovations when they entered Newton High School, where the inter-faith vigil was being held - as did Mr Obama.
"We needed this," Rev Matt Crebbin, senior minister of the Newtown Congregational Church, said.
"We need to be together here in this room. We needed to be together to show that we are together and united."
The children who died were: Charlotte Bacon, 6; Daniel Barden, 7; Olivia Engel, 6; Josephine Gay, 7; Ana Marquez-Greene, 6; Dylan Hockley, 6; Madeleine Hsu, 6; Catherine Hubbard, 6; Chase Kowalski, 7; Jesse Lewis, 6; James Mattioli, 6; Grace McDonnell, 7; Emilie Parker, 6; Jack Pinto, 6; Noah Pozner, 6; Caroline Previdi, 6; Jessica Rekos, 6; Avielle Richman, 6; Benjamin Wheeler, 6; and Allison Wyatt, 6.
The teachers were: Rachel Davino, 29; principal Dawn Hochsprung, 47; Anne Marie Murphy, 52; Lauren Rousseau, 30; school psychologist Mary Sherlach, 56; and Victoria Soto, 27.