The incident happened at Westgate Sports Centre around 9.15am.
Police enquiries are on-going to establish exactly what happened but, at this time, it is not believed to be a terror incident.
Emergencies services are currently at the scene and six casualties have been confirmed.
Police have arrested a 42-year-old woman who is currently in police custody.
Any witnesses or anyone with any information please contact Northumbria Police on 101 quoting reference number 277 25/06/17