A member of the Police Service of Northern Ireland (PSNI) has been suspended from duty while the use of CS gas in Derry is investigated.
The spray was used during disturbances in a car park in the city's Victoria Street area on Saturday night.
It was used again shortly afterwards in the Waterloo Place area apparently to quell fighting among another crowd of youths.
The police officer has been suspended while an internal investigation takes place.
In August, the Police Service of Northern Ireland asked the Ombudsman to investigate all incidents when the spray has been used.
A Police Ombudsman spokesman said: "We are currently investigating all discharges of CS spray by the police following a request by the Deputy Chief Constable .
"We will obviously be dealing with this particular incident in the same way."
A police spokeswoman said the spray was a valuable instrument used to bring a violent individual under control.
"Its objective is to ensure the safety of that person, the protection of the officer and members of the public."
It was the eighth time the spray has been used in Derry since its introduction in the summer. It has been used 11 times in other areas of the province.