Armed PSNI men seized four mortar bombs just minutes before dissident republicans planned to blitz Londonderry's main police station.
They rammed a Citroen Berlingo van, which had been heading for the Strand Road HQ a mile away with four primed home-made missiles ready to be launched through a roof which had been cut open.
Police chiefs admitted the terrorists came dangerously close to inflicting massive casualties.
Chief Superintendent Stephen Cargin said: "There is no doubt about their intention. They were intent to kill and cause maximum police fatalities."
Three men, all known to police in Derry - this year's UK City of Culture - were arrested.
One had been driving the van. A second had been following behind on a motorcycle and the third was detained two hours later in the city's Creggan estate. Two are aged 37, the third 35.
Officers had mounted a major surveillance operation and tailed the Dublin-registered van on the Letterkenny Road before moving in to intercept it in the Brandywell district on Sunday night.
More than 100 homes had to be evacuated and families moved out while Army explosives experts examined the mortars.
The devices had been recently constructed and were similar in design to the type of bombs manufactured and used with such devastating effect by the Provisional IRA before they called a halt to their terrorist campaign in July 2005. Nine officers were killed when a police station in Newry, Co Down, was hit in a missile attack in February 1985.
Mr Cargin, the district commander in Derry, said many civilians could also have died had the attack not been foiled - not just in flats and houses in close to the station at Strand Road, but as the bombs were being transported through heavily built-up urban areas.