A viable pipe bomb was discovered close to a war memorial in Omagh before a Remembrance Sunday parade took place, police have confirmed.
The march was diverted and the traditional wreath-laying at the Co Tyrone town's cenotaph was postponed after the area was sealed off following the discovery of the device yesterday.
It comes almost two decades after a dissident republican Real IRA blast killed 29 in the busy market town in 1998.
Police Service of Northern Ireland Chief Constable George Hamilton described it as "a sickening and appalling act".
He said: "This small but potentially dangerous device was left to cause the maximum amount of disruption to the Remembrance Sunday commemorations.
"This is the action of a small and callous group of violent people who have nothing to offer our communities other than fear and intimidation.
"Whilst our investigation into the incident is at a very early stage, one strong line of inquiry is that violent dissident republicans are responsible.
"Their actions today have demonstrated the disregard and disrespect they have for this community, which has already suffered so much pain and hurt at the hands of terrorists."
The 1998 attack was one of the bloodiest killings of a conflict which lasted for 30 years and happened months after the Good Friday Agreement, which largely ended violence.
The bomb alert also came exactly 30 years after 12 people were killed by an IRA bomb in Enniskillen in Co Fermanagh.
Democratic Unionist Party leader Arlene Foster tweeted: "On a day we remember the carnage of Enniskillen 30 years ago it is disgusting that Remembrance Sunday in Omagh was disrupted by those who left a suspicious device in the town."