More than 20 petrol bombs have been thrown during a third consecutive night of violence in Londonderry.
Most of the petrol bombs and stones were thrown across the city's historic walls from the Bogside area.
There was a strong police presence on and inside the walls, and police officers in riot gear were on standby.
PSNI Supt Gordon McCalmont said that while "extremely young teenagers" were involved, he believed the violence was orchestrated.
"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," he said.
"No one wants to see these despicable scenes on our streets."
Police said between 30 and 40 people took part in the disorder on Monday evening.
A total of 24 petrol bombs were thrown in the area of the city walls and police were attacked with bricks and bottles. Paint was also thrown.
A makeshift barricade was erected and set on fire in the Fahan Street area of Derry.
A laser pen was also shone at the PSNI helicopter.
Supt McCalmont said anyone with influence should use it 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," he said.
No arrests have been made but police said a "significant amount of evidence" had been gathered in recent days.
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Dozens of people, including tourists and some residents from the nearby Fountain estate, came on to the walls to watch the disorder below in the Bogside, reported BBC News NI's Kevin Sharkey.
"But with some missiles landing on the walls, the police eventually ordered onlookers away from the area," added our correspondent.
Why has there been violence?
The violence comes amid heightened tensions as the city marks 50 years since the Battle of the Bogside saw British troops deployed to Northern Ireland.
On Saturday, PSNI officers escorted Clyde Valley Flute Band during a loyalist parade in Londonderry; its bus was later stopped by police.
Band members wore Parachute Regiment insignia during the march bearing the letter 'F'.
At the same time as the Apprentice Boys parade, dissident republicans held a protest in the city centre.
Later this week, an annual bonfire will be lit in the area: Bonfires are lit in some nationalist areas each 15 August to mark the Catholic Feast of the Assumption.
Fears violence will continue
Monday night's violence was the third consecutive night of disorder in the city.
Two petrol bombs were thrown over its historic walls on Saturday night, police said.
As many as 20 petrol bombs and other missiles were then thrown at officers who responded, police added. A number of pallets were also set on fire.
There is concern in the community that the disorder will continue.
"It's an outrageous situation, one that the people of the Bogside are completely opposed to, people are worried it will continue," said SDLP Councillor John Boyle.
"It's disrupting their daily lives, but more importantly, it is threatening people's lives," he said.