Police officers were attacked during an 'unnotified parade' in the Tullyally area of Derry last night.
Masonry and two bottles filled with paint were thrown at officers at about 9.30pm.
No officers were injured, but a number of cars in the area were damaged during the disorder, police have said.
Inspector Mason said police gave verbal warnings to parade participants, after no formal notification of the intention to parade was provided to police.
"Organisers of parades are required to give formal notification of their intentions and so when the un-notified parade formed, police officers administered a number of verbal warnings to participants and an evidence-gathering operation implemented," he said.
"We will now review all the evidence gathered to identify those involved."
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