The Police Ombudsman is investigating after a man was left seriously ill in hospital after police used CS spray during his arrest.
Officers were called to Great Victoria Street at around 5am on Thursday following reports of a violent altercation between a man and a number of taxi drivers.
The incident happened at the junction of Glengall Street and Great Victoria Street.
Police carried out first aid on the man until paramedics arrived on the scene.
The man was taken to Belfast's Royal Victoria Hospital for treatment. He remains in a serious condition in the intensive care unit.
It is thought he is a Belfast man and CS spray was used on him during the arrest outside the Grand Opera House.
A police cordon was put in place at the scene.
The Police Ombudsman's office has been called in to investigate.
Speaking from the scene, senior Police Ombudsman investigator Paul Murphy said: "It is believed CS spray was used during the arrest of the man and at some point during that arrest that man's condition deteriorated.
"Because somebody has been arrested - and is in effect in police custody - and has suffered a potential injury and is critical in hospital it is a matter for the ombudsman to carry out an investigation and to establish the facts of the arrest and if CS Spray has been used and the potential use of force."
The investigator said evidence was being collected and statements taken. He appealed for any witnesses to contact his office.
Police said they would not be commenting further and have referred the press to the ombudsman.
Traffic disruption was caused after the citybound section of Great Victoria Street was closed for much of the day. The road has now reopened.
Bus and train services at the Europa train station were unaffected while the Grand Opera House said it was unaffected and its 2.30pm matinee of Chicago went ahead as normal.
It was also business as usual at the Europa Hotel.