Two men are being questioned in Derry in connection with the discovery of a massive bomb, twice the size of the one used in the Omagh bombing.
Ireland.comalso understands that two men in their 20's are assisting gardaí with their inquiries into the discovery of the 1,200lb bomb in the back of a van yesterday.
The two men in the Republic are understood to be in their 20s and from Co Donegal. They are being questioned at Letterkenny Garda Station.
Police in the North said they believe the bomb, which was primed and ready for use, was the work of dissident republicans.
They have are still questioning two men aged 33 and 24 after arresting them this morning.
Sinn Féin Mid-Ulster MP Mr Martin McGuinness today condemned those responsible for the device.
"The people . . . are involved in these activities because they're opposed to the peace process and they're trying to destroy the peace process," he said.
The Northern Ireland Secretary, Mr Paul Murphy, said the quick actions of the police had prevented a "very significant loss of life.
"My praise for the PSNI officers involved is matched only by my contempt for those who planned to cause chaos and misery".
Analysts said the planned attack was probably timed to sow confusion ahead of a meeting of the ruling council of the Ulster Unionist Party where Mr David Trimble faces a challenge from hardliners opposed to the Belfast Agreement.
Additional reporting PA