Part of an operation targeting dissident republicans was aimed at foiling a plan to procure a significant quantity of arms, police have said.
Ten people were arrested in Monday's raids which saw 200 officers, supported by soldiers, take part in searches in counties Armagh and Fermanagh.
PSNI detectives were also present in France to assist French police in conducting a related search.
One of those held is being questioned in connection with the 1998 Omagh bomb.
Eight premises were searched in Northern Ireland.
Earlier police said they believed they had disrupted a "potential major terrorist conspiracy" in the UK and Europe.
The searches were part of a joint investigation "between the PSNI Crime Operations Department and the British Security Service", said the police.
Detective Chief Superintendent Norman Baxter said the investigation had been ongoing for a number of months and involved close liaison with members of "the Security Service and police services across Europe".
"The investigation has drawn on all branches of the Police Service of Northern Ireland's Crime Operations Department and officers from that department have travelled to Europe to progress the investigation.
"We believe that this operation has disrupted a potential major terrorist conspiracy," Mr Baxter said.