A remembrance service is due to take place later, ahead of the 20th anniversary of the Omagh bombing.
The event will take place in the town’s Memorial Garden.
It was carried out by the dissident republican Real IRA, several months after the signing of the Good Friday Agreement.
The interdenominational service will feature the Omagh Community Youth Choir and include a song specially composed by its musical director, Daryl Simpson.
Michael Gallagher is chairman of the Omagh Support and Self Help Group which is organising the service.
His 21-year-old son, Aiden, was among those killed in the blast.
Mr Gallagher said he had no doubt that the “people of Omagh and beyond” would “come out and show solidarity with the families” on Sunday.
“We have had tremendous support and it is testimony that we are here today as a result of that support,” Mr Gallagher said.
“We would say to everyone ‘please come’, and I am sure you will get something from this service.
“We will try and bring an atmosphere of calm and we are having some special guests that will add to that.”
The Omagh bombing inflicted the greatest single loss of life of any terror atrocity in the history of the Northern Ireland Troubles.
The bomb packed with 225kg of explosives detonated in a vehicle parked in the middle of the main street just after 15:10 BST on 15 August 1998.
A warning had been called in 40 minutes earlier but had given the wrong location of the car containing the bomb.
The dead included three generations of one family.
No-one has been convicted over the bombing.
Omagh bomb timeline
- 15 August 1998 – A large car bomb explodes on a Saturday afternoon in the centre of Omagh, County Tyrone, fatally wounding 29 people.
- 18 August 1998 – The Real IRA claims responsibility for the bomb.
- 6 August 2003 – Alleged founder and leader of the Real IRA Michael McKevitt is found guilty of directing terrorism.
Last year, relatives of the victims announced they would sue the PSNI’s chief constable for failings they believed allowed the killers to escape justice.
On Wednesday, the 20th anniversary of the bombing is due to be marked in the town by a public vigil of “remembrance and hope”.