Police in Londonderry were attacked with dozens of petrol and paint bombs overnight.
Between 30 and 40 people were involved in orchestrated disorder which lasted from early evening into the night, a senior officer said.
A small barricade was erected and burned in Fahan Street, on the edge of the nationalist Bogside housing estate.
Police Service of Northern Ireland (PSNI) Superintendent Gordon McCalmont said: “This is the third night that the people of this city have had to put up with unwanted disorder and destruction.
“No-one wants to see these despicable scenes on our streets.
“Some of those involved in last night’s trouble were extremely young teenagers, and I have no doubt that this disorder is being orchestrated by much older people and that these youths are being used to attack police and significant symbolic sites around our city walls.”
A total of 24 petrol bombs and six paint bombs were thrown in the streets in the vicinity of the historic walls, which separate the surrounding area from the city centre, and police patrols came under attack from youths throwing missiles including bricks and bottles.
A laser pen was also shone at the PSNI helicopter, which police said endangered the crew and local residents.
Mr McCalmont added: “I would urge those young people to pull back from this behaviour and I would also ask those with influence within our community to use that influence positively and help dissuade young people from participating in public disorder.
“It must be made clear to them that they are risking their safety and that of others, as well as running the risk of a criminal conviction which could have a long-lasting impact on their future prospects.
“We have gathered a significant amount of evidence over recent days and I am confident that arrests will follow.”
The violence follows a dispute over a weekend march by the Apprentice Boys loyal order in the city.