A van bomb packed with 600lbs of homemade explosives and abandoned near the Irish border could have caused substantial damage and major loss of life, police have said.
A senior officer said the bomb was primed and "ready to go" and could have had a devastating impact if it had detonated.
The white Citroen Berlingo van was spotted by a member of the public on Thursday evening after it was abandoned on a border road outside Newry in Northern Ireland.
The security operation on the vehicle only ended late on Friday night and District Commander Chief Superintendent Alasdair Robinson said it could have easily claimed lives.
Dissident republicans opposed to the peace process are being blamed for the failed attack and there is speculation the device could have been destined for a town centre or a security target.
The same groups have been linked to the discovery of a smaller explosive device, plus a weapons cache, in north Belfast on Friday night.
The incident was widely condemned, with unionists expressing concern over dissident activity and Sinn Fein calling for the groups to end their campaign which the party said had no support.
Newry's courthouse was damaged by a major car bomb in 2010, but Mr Robinson said the latest find was twice as big as the bomb used in that attack.
The alert was raised when the vehicle was spotted abandoned, with its engine running, on the Fathom Line road outside Newry on Thursday.
Mr Robinson said the likely target, and the reason for the device being abandoned, were unclear.
But the senior officer said the presence of regular police patrols had forced bombers to abandon devices in the past.
He said police had sealed off the cross border road within 12 minutes of being alerted to the suspicious vehicle.
But while some drivers had inadvertently driven past the bomb, others had continued along the route despite police warnings.
The van and bomb have now been taken away for further forensic examination.
Ulster Unionist Danny Kennedy, who represents the area and who is also a minister in the Stormont government, said he believed the bomb was part of a plot to kill police.
The Stormont regional development minister said he was very concerned at the increasing number of attacks planned and executed by republican dissidents around the border town of Newry.
Mr Kennedy paid tribute to the army bomb disposal team and the efforts of the police in dealing with the potentially lethal device.
But he said he feared it would be "only a matter of time" before the violent groups would claim the further lives.
He appealed for the public to co-operate fully with the police.
Sinn Fein's Packie McDonald accused those who abandoned the primed bomb on what is a busy road as being devoid of any political principle.