The scene of devastation after a bomb explodes in Omagh on Saturday, August 15, 1998 killing 29 people, including two unborn twins
A total of 29 people, including a mother pregnant with twins, were killed by the bomb planted by the Real IRA.
After the service in Omagh library, prayers will be said at the scene of the bomb on Market Street.
Flowers will be laid at the glass obelisk at 3.10 pm, the time car bomb exploded.
Tuesday’s event is being organised by the support group Families Moving On.
It follows a public service on Sunday which took place in the town’s memorial garden.
The Omagh families have a led a long campaign in a bid to bring those responsible for the bombing to justice, but no-one has ever been convicted over the attack.
In February, Secretary of State Chris Heaton-Harris announced the setting up of an independent statutory inquiry into the preventability of the Omagh bombing.
In a statement on the 25th anniversary, Mr Heaton-Harris said news of the bombing of Omagh “reverberated around the world” in August 1998.
“While responsibility for this appalling crime lies with the murderers and those that assisted them, it is important that all lessons are learned and that confidence in this is given to the families of those affected, and to wider society,” he added.
