Police have said that a bomb found at the border near Newry contained 600 pounds of explosives and was fully primed.
The device, which had been placed in an abandoned van on the Fathom Line, was discovered on Thursday.
It was made safe on Friday evening.
Ch Supt Alasdair Robinson said the bomb was twice as big as the bomb that exploded outside Newry courthouse two years ago.
"If this had exploded it would have caused devastation," he said.
"To put it in perspective - anyone within 50m of this device would have been killed and anyone within 100m, seriously injured."
The police chief also rejected criticism that motorists had been able to drive past the bomb and said police had closed a main cross-border road within 12 minutes.
"Many police services would struggle to get an international border road closed in this time frame," he added.
"There was some residual traffic following the closures and, while we made the public aware of the road closures and advised them to avoid the area, a few cars ignored our advice.
"We do everything in our power to protect human life."
The Fathom Line was closed for 24 hours while the police and Army dealt with the alert.
Ulster Unionist MLA for the area, Danny Kennedy said if the bomb had exploded it would have caused serious destruction.
He said: "It had the potential to cause lethal damage.
"A 600 pound device at the road side waiting for a police patrol. It is just unthinkable."
A member of the public reported the discovery of a suspicious vehicle to police on Thursday.
It had been abandoned with its engine running.
Dissident republican paramilitaries have been blamed for a number of bomb attacks in the city in recent years.
Earlier this month, a bomb was found near the Cloghogue roundabout in Newry, just off the main Belfast to Dublin dual carriageway.
Police said the device contained a significant amount of explosives and had the potential to kill.
It was also made safe by army bomb experts.