Donna-Marie McGillion suffered over 60% burns in the 1998 Omagh bombing and was forced to delay her wedding while she recovered.
The victims came from Northern Ireland, the Republic of Ireland, England and Spain.
An inter-denominational service is take place at a memorial garden in the Co Tyrone town.
The father of one of the victims of the 1998 Omagh bomb attack has hit out amid reports of an imminent collapse of the prosecution of a man accused of the murders.
Michael Gallagher, whose son Aiden was among 29 people to be killed in the Real IRA bombing, expressed outrage that families have been kept in the dark.
The case against 45-year-old County Armagh bricklayer Seamus Daly - who denies involvement in the bombing - will be mentioned at a court in Ballymena later this morning.
Court proceedings are currently subject to reporting restrictions.
Mr Gallagher said he was unhappy that information was circulating about the collapse of the case while the families had not been notified.
"We have been failed once again by the police service, by the prosecution service, by the government and by the criminal justice system," he said.
High-profile republican Seamus Daly has been charged with murdering 29 people in the Real IRA Omagh bombing of 1998, the Police Service of Northern Ireland said.