Petrol bombs have been thrown at police during trouble in Warrenpoint in County Down on Tuesday night.
A group of up to 40 youths gathered in Duke Street at about 2130 GMT.
A number of cars were damaged. No-one was arrested. The police said CCTV footage would be examined and any offenders identified would be arrested.
UUP assembly member Danny Kennedy condemned "the widespread anti-social behaviour which affected significant areas of Newry and Mourne".
Mr Kennedy said "rogue elements" had attempted to "lure police" by using the Halloween festival as an excuse to attack property, individuals and the PSNI.
He described the scene in Bessbrook village in County Armagh as "something akin to Beirut, with bottles, bricks and large lumps of wood and burning embers scattered" all over the centre.
Sinn Fein councillor for Newry and Mourne Mickey Ruane said the incident was "completely unacceptable".
"This type of outrageous behaviour can in no way be excused and indeed it is only by luck that no-one was seriously injured," he said.