A letter bomb has been found at Northern Ireland's main postal sorting office in Northern Ireland.
It was recovered at the distribution centre at an industrial estate in Newtownabbey, near Belfast.
The device marks the re-emergence of a terror tactic formerly used by paramilitaries during the Troubles.
Dissident republicans opposed to the peace process have used packages in the recent past to target high-profile figures including Northern Ireland Secretary Theresa Villiers.
The alert was declared as Northern Ireland prepares to mark the culmination of the unionist marching season - a time when the Police Service of Northern Ireland (PSNI) is mounting a massive policing operation and major efforts have been made to ensure a fragile peace is kept.
A PSNI spokeswoman said: " Police are currently attending a security alert following the discovery of a suspicious package at the postal sorting office in Mallusk Road, Newtownabbey."
Army bomb disposal experts were sent to the scene.
A series of letter bombs sent to Army recruiting offices in England have been blamed on dissidents.
In October last year dissidents also sent a series of explosives to high-profile political and security figures in Northern Ireland.
One was addressed to the seat of the power-sharing executive at Stormont Castle in Belfast, addressed to Northern Ireland Secretary Theresa Villiers.
Another was delivered to the office responsible for court prosecutions in Derry while two explosive packages - one addressed to former PSNI chief constable Matt Baggott and the other to one of his senior commanders - were intercepted at Royal Mail offices in Belfast and Lisburn.
Nationalist SDLP councillor Noreen McClelland said those responsible for the latest device were reckless thugs who put countless lives in danger.
"This was a cruel, callous act that could have left people injured or dead. Those behind it have no regard for the lives of anyone caught up in their attempts to maim and kill others."