A 12-year-old boy was among a trio of youths accused of assault at Newport on Sydney's northern beaches.

A stunned senior Sydney police officer has penned a powerful plea to parents about the rising tide of violence and drug use among hundreds of teenagers on the city’s northern beaches.

Superintendent Dave Darcy told news.com.au the problem had been “festering” in the idyllic coastal community for a while but had been “getting worse and worse” in the past two months.

He said police officers and members of the public were being attacked, as a core group of around 200 teenagers dabble in drugs such as valium, xanax and methamphetamine.

“They’re buying pills which they think is ecstasy, but it’s really ice, and it’s making them aggressive,” Supt Darcy said.

“They are attacking our officers and leaving them with injuries. That’s unusual and it shouldn't happen.”

In a letter he sent to parents through local schools in the northern beaches, Supt Darcy wrote there had been a “significant deterioration in the behaviour of young people” — who use Facebook to arrange meet-ups that turn nasty.

“Teens, ages ranging from 13 to 18, from local schools are out all hours of the night, fuelled up on alcohol and drugs, both illegal and prescription,” he wrote in the letter, seen by news.com.au.

He warned parents police officers were wearing high-definition cameras, and he was shocked by what he had seen cops record.

“I have seen this video and, even with more than three decades as a police officer, I find the levels of anger and aggression some young people are bringing to the streets to be particularly confronting,” he wrote

“I am also surprised at the freedom enjoyed, particularly by the 13 and 14 year olds who are roaming our streets at midnight under the influence of alcohol or drugs.

“It’s behaviour that’s escalated from the problematic to, in some cases, the criminal. People, including police, are being attacked and injured.”

A vicious fight involving 150 teenagers broke out outside a McDonald’s restaurant in Warriewood, on the Northern Beaches, about 11.30pm on May 4. Some of the children involved in the brawl had to be subdued with capsicum spray.

A witness told The Daily Telegraph the fight started when two girls had a disagreement.

“It was absolutely wild, a lot of them were really drunk … I reckon there was about seven girls I saw sprayed. Parents were running in with water bottles to wash their eyes out,” the witness said.

In March, an off-duty police officer was caught up in a wild street fight on the northern beaches that resulted in three boys being led away in handcuffs.

A group of passers-by tried to break up a fight between boys in Newport before being attacked themselves.

“Girls were screaming, and there were numerous young people milling about, some wearing school uniforms,” a neighbour told The Manly Daily.

“I was told that an adult stepped in to break it up, and the kids turned on him before the brawl escalated.”

However, in his letter to parents, Supt Darcy said those teenagers taking part in the violence and drug taking were fighting a losing battle.

“Our ability to identify, find and charge those committing criminal acts has improved markedly (due to body cameras and audio recording equipment),” he wrote.

“If we don’t stop them on the night, we will use the video and audio captured on these cameras to determine who they are, where they live and come knocking on their parents’ door.”