An improvised weapon has been discovered in the playground of a primary school in north Belfast, the police have said.
It was discovered at Holy Cross Boys' Primary School and was "most likely" left by dissident republicans to attack police, a senior officer said.
Ch Supt Jonathan Roberts said the weapon had the "potential to fire a high-calibre round".
"What they have done is endangered the lives of children," he said.
"The device was left in the immediate area where the youngest children who attend school would be playing during the course of the school day."
He said it was an "act of utmost recklessness".
The weapon was found by the principal and caretaker under a sewage man-hole cover, in a plastic package in a garden area of the school.
The school was evacuated on Monday and will be closed again on Tuesday.
Ch Supt Jonathan Roberts added: "Our working theory at this time is that this weapon was most likely to have been left there by dissident republican terrorists.
"It was probably destined to be used in an attempt to kill or seriously injure police officers who are serving and protecting the community of north Belfast."
Speaking to BBC Radio Ulster earlier, the school's principal, Kevin McArevey, spoke of his shock.
"I was helping the caretaker with some sewage problems out the back of the school, in the nature garden and we had to lift a manhole cover to get the rods down and to my surprise, there was a plastic package just sitting in the sewers.
"I lifted it out and... when I opened it up, there were wires at the top of this and tubing in it.
"It was a scary moment for both of us."
He added: "We had cleared out the sewers three years previous, it was put in there within the last three years.
"Whoever left this device should consider their reckless disregard for the health and wellbeing of the children who would regularly use the nature garden."
In a tweet, North Belfast MLA Nichola Mallon said the incident was a "disgrace".