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The blast in the market town of Omagh, County Tyrone, at around 1500 BST on Saturday, left more than 100 people injured or maimed.
People who survived the car bomb blast in a busy shopping area of the town have been describing scenes of carnage with the dead and dying strewn across the street.
Police received a telephone warning approximately 40 minutes before the blast.
But the location of the bomb was unclear and the wrong area was evacuated, with people being directed towards the danger zone.
By 1830 BST the number of confirmed dead had risen to 21.
Political leaders have been joined by the Queen in expressing their sympathy for the bereaved and those injured in the explosion.
Martin McGuinness, the chief negotiator for Sinn Fein, said: "This appalling act was carried out by those opposed to the peace process.
"It is designed to wreck the process and everyone should work to ensure the peace process continues."
Ulster Unionist security spokesman Ken Maginnis described the bombing as "a dreadful crime against humanity".
No group has yet admitted planting the bomb which was found to have been planted in a maroon Vauxhall Astra.
Northern Ireland's police chief Ronnie Flanagan said they would be focusing their attention on a republican splinter group calling itself the "Real IRA".
He said: "It is possible and probable that they carried out this attack.
"These are people who have murdered here today because they want to murder."
In Context
The death toll eventually reached 29. The dead included nine children and a woman pregnant with twins.
The Real IRA - a breakaway group formed after the Provisional IRA declared a ceasefire - admitted planting the bomb.
In a statement the organisation apologised to the civilian casualties claiming its intended targets had been "commercial".
Its subsequent announcement of a "suspension of violence" was rejected by politicians on all sides.
In January 2002 Colm Murphy was sentenced to 14 years in jail for conspiracy to cause an explosion. But his conviction was overturned on appeal in January 2005 and he faces a retrial.
Sean Gerard Hoey of South Armagh is the only man charged with murder in connection with the bombing. His trial began in September 2006.
On 6 August 2003 Michael McKevitt, leader of the Real IRA, was convicted of directing terrorism and membership of an illegal organisation and given a 20-year jail term.
In November 2005 it was revealed that victims of the Omagh bombing had been paid more than £20m in compensation.