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A bell will ring in Omagh later to mark the moment a car bomb exploded 20 years ago killing 29 people, the greatest single loss of life in the Troubles.
A woman pregnant with twins was among those killed by the dissident republican attack in the County Tyrone town on 15 August 1998.
The bell will be rung 32 times in memory of the victims.
The additional, single peal will be rung for all who have lost their lives in atrocities around the world.
The Omagh bombing was carried out by the dissident republican Real IRA, several months after the signing of the Good Friday Agreement.
Omagh bomb timeline
The bomb, which was 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.
The bell-ringing event is part of a public vigil to be held at the bottom of Market Street, beginning at 14:55.
The bell will stop ringing 15 minutes later, at the time of the explosion, and will be followed by a two-minute silence.
Flower petals will be distributed, which people can scatter in the river, or place in the pond of a memorial garden created to remember the victims.
These event is being co-ordinated by a group of organisations, including Omagh Support & Self Help Group, Families Moving On and the Omagh Churches' Forum.
Victims organisation Omagh Families Moving On is chaired by Kevin Skelton, whose wife, Mena, was killed in the attack.
Mr Skelton said the event was about remembrance, hope and moving forward, recognising the forgotten people of the Omagh bomb including those involved in the aftermath of the bomb such as shopkeepers and undertakers.
On Sunday, a cross-community service was held at the town's memorial gardens.
Victims and their families were remembered with prayers, music and speeches.
Wreaths were laid and the names of all those killed in the attack were read out.
Each year over the past 20 years, people have come together to mark the anniversary, but this year's event in the memorial garden will be the last to take place on this scale.
Last year, relatives of the victims announced they would sue the PSNI's Chief Constable George Hamilton for failings they believed allowed the killers to escape justice.
Mr Hamilton said on Sunday he understood why the families would feel "angry and let down", adding that even the huge amount of investigative effort - with 99 arrests and 11,000 investigative actions by the PSNI and An Garda Síochána (Irish police) - "is not good enough".
"People have not been brought to justice... but the families have an assurance from me that if new evidence emerges, we will actively pursue that. But it is also fair to say, and realistic, that as time goes by, the chances of a criminal justice outcome reduces," he said.