Letter-bomb was addressed to Chief Constable George Hamilton at the PSNI's HQ
A letter-bomb sent to the PSNI's headquarters was addressed to Chief Constable George Hamilton, sources have revealed.
It is believed that the viable device made safe by the Army was posted from the Republic.
The alert was raised after a suspicious package in a brown padded bag with a white address label was received by post at the east Belfast headquarters.
PSNI Superintendent Sam Donaldson said someone could have been seriously hurt.
"This is a mindless and foolish act that could have resulted in the serious injury of any person handling the package," he said.
"Although we have no information to suggest there may be other similar packages in the postal system, I would encourage people to be vigilant."
Alliance East Belfast MP Naomi Long condemned those behind the attack.
"There can never be any justification for such acts of terrorism," she said.
"It was clearly the intent of those who sent the device to seriously injure or kill someone.
"Sending a letter-bomb is a reckless and indiscriminate act and those behind it are beneath contempt.
"I would like to praise the brave actions of the police and Army officers who dealt with this device.
"Their bravery stands in stark contrast to those evil individuals behind this bomb.
"The vast majority of people in Northern Ireland are opposed to the actions of those who sent this device.
"Those responsible must be caught by the police, so I would urge anybody with any information to contact the PSNI or Crimestoppers."
Last year a group calling itself the IRA said it had sent letter-bombs to Army recruitment offices across the UK, marking the resurgence of a terror tactic adopted by paramilitaries during the Troubles.
Four suspected explosive devices were discovered at Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough.
Packets were also sent to military careers offices in Aldershot in Hampshire, Reading in Berkshire and Chatham in Kent.
One of the devices was sent to the seat of the power-sharing Executive at Stormont Castle in Belfast, addressed to the Secretary of State, Theresa Villiers.
Another bomb was delivered to the offices of the Public Prosecution Service in Londonderry, while two explosive packages - one addressed to then PSNI Chief Constable Matt Baggott and the other to one of his senior commanders - were intercepted at Royal Mail offices in Belfast and Lisburn.