Four live mortar bombs were intercepted by police in Northern Ireland minutes before being launched, a senior detective has said.
A van had its roof cut back to allow the mortars to be fired.
Police believe the target was a Londonderry police station. Three men have been arrested in the operation linked to dissident republicans.
Chief Supt Stephen Cargin said the bombs could have caused mass murder.
Two men, aged 35 and 37, were arrested at the scene at 20:15 GMT on Sunday. One was in the van and the other was driving a motorcycle travelling behind.
A 37-year-old man was arrested later after a house was searched.
It is the first time dissidents have attempted this type of mortar attack.
About 100 families had to leave their homes at Letterkenny Road in the overnight alert.
Chief Superintendent Stephen Cargin said there could have been mass murder if they had hit the intended targeted.
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Thank goodness they stopped this from happening. I wonder if someone tipped them off as to what was going to happen. If so, that person is a hero in my book.