Four men and two women are today being questioned by police investigating the financing of a transatlantic IRA gun smuggling operation.
They were held after swoops on houses in Belfast and North Antrim.
Officers carried out raids in the latest phase of an investigation in the US, Britain and Ireland into huge arms importation by the Provisionals after the ceasefire was restored.
The arrests will heighten pressure for the IRA to disband thereby reviving power-sharing.
Some of the smuggled rifles, shotguns and pistols are still missing but security sources last night revealed the focus at this stage in the three-year investigation is on how the operation was funded.
"This is more to do with financing rather than the guns. Who knows where the weapons are hidden?" one source said.
Sinn Fein immediately accused Special Branch of trying to undermine the peace process, but unionist MPs insisted the case for the IRA to prove its campaign was over once and for all was now greater than ever.
Ulster Unionist Lagan Valley MP Jeffrey Donaldson said: "Sinn Fein/IRA must deliver an end to this kind of activity."
Police in the Republic, the FBI and Scotland Yard have also been involved in thwarting attempts by the Provisionals to secretly import a consignment of guns.
The operation which led to yesterday's arrests dates back to 1999 when guns and component parts were discovered by postal staff in Britain.
Police in Belfast said the weapons had been posted from the US, often in boxes containing toys, before being intercepted in England.
As part of the operation, four people were later convicted in Florida of firearms offences in September 2000. Later a number of the weapons turned up in the Republic where four men were jailed for guns offences last month.
A spokesman for the Police Service of Northern Ireland confirmed the force had now taken the lead role in the investigation. "Some of the weapons believed to have been bought in Florida, including rifles, shotguns and handguns, are still outstanding," he said.
"Around 100 weapons and ammunition have been transferred by law enforcement agencies in America to the PSNI as part of the investigation."
The arrests will be a new blow to Sinn Fein as it rejects allegations that the IRA-trained Farc Marxist guerrillas in Colombia, stole Special Branch files from Castlereagh police complex and was involved in spying at the Northern Ireland Office.
PA