A man was taken to hospital after he was found with a serious injury in the street.
Emergency services were called to Suffolk Street in Hendon, close to Fletcher's Off-Licence, just before 2pm today after reports a man was hurt and damage had been caused to the store.
North East Ambulance Service paramedics were on the scene within six minutes and took the man, who was wearing a Celtic top, to hospital.
Medics found he had sustained a serious head injury, but is not in a life-threatening condition.
A spokesman for Northumbria Police said: "At approximately 1.40pm today, police received reports of an incident at Fletcher's Off-licence in Hendon.
"Police and paramedics attended the scene after damage was caused to the off-licence and allegations of assault were reported.
"A man has been taken to hospital with a head injury. Police enquiries are ongoing to determine the circumstances around the incident.
"Police are reminding all Sunderland and Celtic fans that any form of disorder, violence or any behaviour that endangers the public will not be tolerated and police action will be taken."
Up to six other ambulances were dispatched in the run up to today's friendly between Sunderland and Celtic to mark the 20th anniversary since the Stadium of Light began to welcome fans.
The service has said they were sent to assist police, but found they were stood down once they arrived at the incidents.