A letter bomb has been found at Northern Ireland's main postal sorting office in Mallusk Road, Newtownabbey.
It was recovered at the distribution centre at an industrial estate near Belfast.
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 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 .
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.
Elsewhere, three men were stabbed in separate incidents in Belfast.
One man, 28, was stabbed near the Ormeau Bridge in the south of the city around 3am.
Police officers remained in the area for some time after.
Separately, a teenager was stabbed in north Belfast at around 1.20am.
The PSNI are seeking two men who left the area in a silver Clio car travelling towards the Antrim Road.
Meanwhile, in east Belfast a 21-year-old man was stabbed in Montrose Street. A man has been arrested in connection with this incident.