Police believe they have targeted the "entire command" of the Continuity IRA following one of the biggest anti-terrorism operations in recent years.
Detectives were last night continuing to question 12 men, aged between 36 and 75, who were arrested at a house in Newry on Monday night.
Highly trained and heavily armed officers from an elite anti-terrorism squad swooped on the house in Ardcarn Park in the city following months of covert investigations into dissident republican terrorism.
It is understood they were acting on intelligence that the Continuity IRA's top officials were meeting at the property to plot a major terrorist attack.
One of those arrested during the security operation was Patrick Joseph 'Mooch' Blair, who has previously been accused of building the Omagh bomb.
Blair was once named in the House of Commons as having "helped to construct the Omagh bomb" and taken part in a string of other attacks. The 58-year-old later denied any involvement with the 1998 atrocity, in which 29 were killed.
The 12 men were arrested in relation to "violent dissident republican activity" under the Terrorism Act and can therefore be held in police custody for questioning for up to 28 days upon a successful application by the PSNI to the High Court.
It is understood the arrests and a number of related searches launched both north and south of the border follow a significant security operation that has been ongoing for a number of months.
Undercover police officers, with the assistance of MI5, are understood to have had members of the CIRA under surveillance for several months. After receiving intelligence that a meeting of senior CIRA members was to be held on Monday evening in Newry, the specialist police unit was placed on standby.
When given the order, dozens of specially-trained officers - heavily equipped with automatic weapons, heavy body armour and high-tech helmets fitted with cameras, goggles and microphones - descended on the bungalow, which is situated in the mainly nationalist Carnagat estate.
"This has been in the planning for some time and was about going after the top command of the dissident group. We believe the entire command was meeting together, and they weren't there to play bingo," a security source said.
Policing Board member Jonathan Craig described the police operation as "hugely significant".
The DUP MLA added: "Such an operation simply would not happen without detailed intelligence work.
"It is vital that the campaign against dissident terrorists is maintained and that we see convictions secured against those who pose a threat to our safety."
One of the men arrested on Monday night, Patrick Joseph 'Mooch' Blair, was once named in the House of Commons as having "helped to construct the Omagh bomb". In 2002 Lagan Valley MP Jeffrey Donaldson used Parliamentary privilege to name Blair in connection with the attack. He said Blair was "a member of the Provisional IRA and who helped to construct the Omagh bomb".