Twenty-nine people were killed in the Omagh bomb
The Omagh bombing was "a massacre of the innocents", a civil action against five men that relatives believe were behind the 1998 attack has heard.
Twenty-nine people died in the Real IRA bomb in the County Tyrone town.
Opening the case at the High Court in Belfast, Lord Daniel Brennan QC, who is representing the families, described it as an "unprecedented civil action".
"For the first time, the victims of terrorism are suing the alleged perpetrators," he said.
"For the first time, private citizens are confronting terrorists in our courts."
Lord Brennan said his case would include evidence from an FBI agent who infiltrated the Real IRA.
He said it would also include evidence about a British sting operation and would involve detailed information from both the police in Northern Ireland and the Irish Republic.
The case is expected to run for up to eight weeks and will transfer from the High Court to the Supreme Court in Dublin to hear evidence from a series of police officers in the Irish Republic.
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.
Last December, Sean Hoey, an electrician, from Jonesborough, south Armagh, was cleared of all charges related to the bombing and a series of other Real IRA attacks.
The families are seeking compensatory, aggravated and exemplary damages against five defendants.
Those being sued are Michael McKevitt, Seamus Daly, Liam Campbell, Colm Murphy and Seamus McKenna.
They all deny involvement in the bombing.
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"
Lawyers representing the defendants objected to Lord Brennan describing their clients as terrorists and also for listing their history of previous convictions.
Their barrister, Michael O'Higgins SC, described it as "utterly prejudicial".
However, the judge let Lord Brennan continue pointing out that what was said in the opening of a case did not amount to fact in law.
Lord Brennan criticised the defendants, not one of whom, he said, had indicated that they were prepared to give evidence during the case.
Mr Justice Morgan said that was an issue he would be addressing on Tuesday morning.
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.