It is only a matter of time before dissident republicans kill police or civilians, it has been claimed.
Just hours after they bombed the offices where preparations are taking place for Londonderry's year as City of Culture, Chief Superintendent Stephen Martin warned of the danger from future attacks.
Doors, shutters and the interior of the Customhouse Street premises were damaged and glass was broken in nearby buildings.
Drinkers were smoking outside a bar just 15 yards from where the rucksack device exploded but the area had been cleared following a warning.
Mr Martin said: "It is only a matter of time until some mistake happens and these people take the life of a police officer or a civilian."
This is the second time the City of Culture 2013 offices have been targeted this year. The office is in Guildhall Square, a busy thoroughfare with restaurants and businesses nearby.
Police received a warning at 9.45pm yesterday and saw the rucksack-type bag lying at the door of the premises. They evacuated the area and it exploded an hour later, before army technical officers arrived to deal with it.
Mr Martin said the bombers were "senseless".
"People, most probably from this city, carried out an attack, not on the police last night, they carried out an attack on the people of this city," he said.
He said someone near the explosion would have been killed.