A man is critically ill in hospital after police used CS spray during a violent incident in Belfast city centre.
The trouble happened near the Grand Opera House at about 5am on Thursday and a defibrillator was left at the scene.
The Police Ombudsman is investigating and said police were called after a man was suspected of involvement in an altercation with taxi drivers.
A spokesman for the office said: "During the incident police officers used CS spray against the man who later became unwell.
"He received medical attention at the scene before being transferred to hospital."
Ombudsman investigators have been at the scene from early morning.
The area was cordoned off and a section of Great Victoria Street was closed for a time but has been reopened.
Investigators have mapped and photographed the scene, security camera footage has been recovered and enquiries are ongoing to acquire further footage.
A number of witnesses have been spoken to but the office has issued an appeal for anyone else who was in the area at the time and may have seen what happened to contact them.