Three children were injured after an explosion in a wheelie bin near a house in Lurgan, Co Armagh.
It is believed that two 12 year-olds as well as a two-year-old child received minor cuts from flying debris and suffered shock when the blast happened.
The PSNI says there was no warning about the device, which went off near a house at Kilmaine Street, off North Street.
As it is close to the main Belfast to Dublin railway line, train services were suspended.
PSNI Chief Inspector Sam Cordner has condemned those responsible as cowards who had no regard for human life.
Mr Cordner said it was an obvious attempt to kill or injure police officers.
SDLP Upper Bann Assembly member Dolores Kelly said the bomb was a despicable act.
She said a call had been made to the Samaritans saying a device had been placed near the Model School, but there was no mention of what town.
Police all over Northern Ireland were out checking all Model Schools when the bomb went off without further warning at Kilmaine Street.
Ms Kelly said it was a despicable attempt to draw police in and then set off a bomb precisely where they would have been trying to keep other people back out of danger, and that is why the children were injured by debris from the explosion.
SDLP chair of Craigavon DPP Councillor Michael McGoldrick said the community was outraged and was pulling together in the face of this threat.
Meawhile, British army bomb experts have carried out a controlled explosion on a suspicious object at a builder's yard in Lurgan.
A number of houses have been evacuated.
A bomb disposal team was called to the Spencer Road in the Waterside area of Derry following a warning call at around 2am that an object had been left there.
A local guest house had to be evacuated. The suspicious object has since been declared a hoax.
The PSNI says five petrol bombs were thrown at police as they attended the alert.
British army bomb experts have carried out a controlled explosion on a suspicious object at Dunmurry Close near west Belfast.
A security alert at Greencastle, near north Belfast has been called off and declared an elaborate hoax. British army bomb disposal experts carried out two controlled explosions at the scene.