Twenty-nine people were killed in the Omagh bomb
A civil action against five people Omagh relatives believe were behind the 1998 bombing has got under way.
Twenty-nine people died and hundreds were injured in the Real IRA bomb in the County Tyrone town.
While no-one has been convicted of the atrocity, some families are suing the men they believe were involved in the attack for more than £10m.
Lord Brennan, who is representing the six families taking the action, said the case was unprecedented.
It was the first time in the UK and probably anywhere in the world that "private citizens are confronting terrorists" in the courts, he said.
The families are seeking compensatory, aggravated and exemplary damages against five defendents.
Those being sued are Michael McKevitt, Seamus Daly, Liam Campbell, Colm Murphy and Seamus McKenna.
They all deny involvement in the bombing.
The court will move to Dublin later in the trial to allow evidence to be taken from Garda officers.
Speaking before going into the court, Michael Gallagher, whose son Aidan, 21, died in the bombing, said: "It's significant the fact that we've actually came here after seven years and I think that it's proof of the fact that both governments have failed us in the justice system.
Six families are taking the case
"It's basically left up to victims to seek justice for themselves, but we think it's an important day, regardless of what the outcome of the trial is."
He said: "I would say to the people that bombed Omagh, we certainly haven't gone away and we will not go away."
Sean Hoey, an electrician from Jonesborough, south Armagh, was cleared of involvement in the Omagh bombing last December.
McKevitt, 58, is awaiting judgement in an appeal against a conviction in the Irish Republic for directing terrorism.
WHAT IS A CIVIL ACTION?
Brought to seek financial compensation - it does not result in a criminal conviction
Case must be proved on the "balance of probabilities" rather than criminal law's requirement of "beyond reasonable doubt"
He has won a fight for full legal aid to defend the multi-million pound compensation case.
Campbell, 43, was held in Portlaoise Prison in the Republic of Ireland on terrorism charges unconnected to Omagh.
Colm Murphy, 56, was sentenced to 14 years in prison in January 2002 for conspiracy to cause the Omagh explosion, but the conviction was overturned on appeal and he is awaiting a retrial.
All five are defending against the action and deny any involvement in the attack.
Part of the court proceedings will be relocated to Dublin in an historic step to allow the judge, Mr Justice Morgan, to hear evidence from 24 garda officers.