The selfless worker noticed the device had ignited in the Golf Centre store in Cornmarket and carried it out before any serious damage could be caused.
No one was injured in the attack
The area remains closed off and Army technical officers are now at the scene.
The PSNI have renewed their appeal to all key holders of shops, bars and restaurants to carry out a thorough check of their premises before locking up tonight.
The same warning was issued after last Friday night’s bomb attack at Exchange Street West in the Cathedral Quarter area.
A device in a holdall exploded outside a block of flats while police were clearing the area.