The public are being asked to report to police immediately if they receive a package in a white A4 envelope with address in grey stencil.
It follows the discovery of a suspicious package at a postal sorting office in the Great James Street area of Foyle.
The PSNI has confirmed tonight (Thursday) that the envelope contained a viable device.
She asked that the public be vigilant and report any such packages arriving in the post or anything suspicious.
Read more news:
Sign Up To Our Newsletter
Sign up to our newsletter for major breaking news stories as they happen – right to your inbox.