Buildings were evacuated and streets closed after reports of an explosion on Victoria Street in Belfast city centre, where police were earlier dealing with a suspect vehicle.
The PSNI had evacuated the area while they examined the car near the Victoria Square shopping centre, the Northern Ireland Court Service and the Musgrave Street police station. A loud bang was subsequently heard.
Victoria Street, Chichester Street, May Street, Oxford Street, Ann Street, Seymour Street were closed to traffic after the incident began on Sunday night at around 9.30pm.
Police said traffic diversions were likely to remain in place for a "number of hours".
A PSNI spokeswoman said: "Police are continuing to deal with a security alert in Belfast City centre. It is likely that this will continue for a number of hours".
Police at the scene said it was not a controlled detonation, the BBC said.
The Police Federation of Northern Ireland said via Twitter: "PSNI and ATO are dealing with a complex and ongoing fluid situation in Belfast. Please bear with us as this could continue for some time."