Almost 40 people have contacted the Police Ombudsman about disturbances during a band parade in South Belfast last month.
A police officer used CS spray during trouble linked to a Junior Orange Lodge march on the Ormeau Road on March 29.
An Ombudsman spokesman said the watchdog was keen to hear from anyone else who may have seen what happened or recorded video footage, whether or not they were involved in the parade itself.
He said: "The incident happened at around 5.10pm close to the Ormeau Road's junction with Park Road, near the Ormeau Bakery apartments.
"We're keen to hear from anyone who saw what happened immediately before, during and after the use of CS spray by an officer."
Loyalists aggrieved at police handling of the march held a protest outside Musgrave police station in the city centre on Tuesday night.
The DUP have distanced themselves from a Ballymena councillor after he shared the image of a police officer being blamed by loyalists for using CS spray at the Junior Orange Order Parade and branded the PSNI "bootboys".
The release of the name and address of a police officer, involved in dealing with disturbances during a loyalist parade on the Ormeau Road, has been condemned by the Police Federation for Northern Ireland.
Chief Superintendent Chris Noble said: “After a busy day of successful and peaceful events across Belfast, unfortunately an incident towards the end of the day, which could have been avoided had some bandsmen followed police directions, has become the news headline.