Dissident republicans are suspected after police discovered two bombs and a cache of weapons during three separate security alerts in Northern Ireland.
The explosives were described as ?viable devices? and were found in Belfast and Newry.
The first bomb alert began in the Fathom Line area outside Newry on Thursday evening after a white Citroen Berlingo van was found abandoned.
Police confirmed a device containing over 250kg of explosives was made safe by army bomb experts in a security operation which ended late last night.
A senior PSNI officer said the device was larger than had originally been estimated and could have caused major loss of life had it detonated.
?The device contained two blue barrels with 125kg of homemade explosives in each one, and a detonator - all the equipment which meant this device was ready to go,? said District Commander Chief Superintendent Alasdair Robinson. ?This was a very significant device. If this had exploded it would have caused devastation.
?To put it in perspective - anyone within 50 metres of this device would have been killed and anyone within 100 metres, seriously injured.?
Local Ulster Unionist MLA Danny Kennedy, who is also minister for regional development in the Stormont government, said the bomb was believed to be part of a plot to kill police.
Mr Kennedy said the scale of the bomb suggested that an attempt at a major attack by dissident republicans had been prevented. ?Clearly this was an attempt to lure a police patrol into that area, with potentially lethal consequences,? he said.
Mr Kennedy said the dissidents were ?dangerous and dedicated terrorists, who are determined to cause serious harm, injury and death to members of the security forces regardless of the consequences to local communities?.
In addition to the discovery of the substantial device in Newry, detectives found a smaller device under a car in the Ballygomartin area of north Belfast, which was also blamed on dissidents. In a third overnight operation, also in north Belfast, detectives investigating dissident republican activity found a number of guns and ammunition.
Several homes had to be evacuated while the security operation was carried out late last night.
?Those responsible for this have shown callous disregard for members of the public," said Chief Inspector Ian Campbell. "The operation resulted in the evacuation of up to 80 people, including families with young children and elderly residents, for several hours.
?The finger of suspicion points towards dissident republican terrorists and I appeal to anyone with information to come forward to police.?
Following last night?s weapons find in Belfast, Chief Superintendent George Clarke, District Commander for North and West Belfast, said the police had succeeded in combating activity by the dissident groups.
?The actions of police have undoubtedly thwarted the attempts of criminals to inflict death, injury and misery on the community of north Belfast,? he said. ?Police are determined to protect communities from these threats.?
Mr Clarke appealed for the public?s continuing help in tackling dissidents.
PA