Two masked men entered the premises, dropped a bag and issued a 45-minute warning at 09:45 BST on Monday.
Hill Street was closed between the cathedral and the town hall and dozens of businesses were disrupted.
In May, a bomb exploded in a Santander bank in Londonderry. It was thrown into the bank by masked men and dissident republicans were blamed.
Appealing for information on the Newry bomb, the area commander for Newry and Mourne, Ch Insp Davy Beck, said: "This was a viable device which was capable of causing death, serious injury and substantial damage to the area.'Investigation'
"I would place on record my gratitude to the army technical officer and his team for their actions which clearly prevented the detonation of this device.
"Newry city centre is returning to normal at this point however a thorough investigation into the attack has been initiated.
"I would appeal to the community for information about the people who conducted this cowardly attack on the people of Newry & Mourne."
Meanwhile, homes have been evacuated due to a security alert at Ozone Avenue in Ballymoney. Police and Army bomb experts are at the scene.
Ozone Avenue and Meeting House Street are closed to traffic. Motorists are advised to avoid the area.