It is only a matter of time before a police officer or a child is seriously injured over violence in Londonderry, the PSNI's chief constable has said.
George Hamilton said that some police officers have sustained injuries.
Police fired plastic baton rounds on Thursday night, which was one of the worst nights of violence in Derry for years.
Six people were arrested, at least one for attempted murder.
Seventy-five petrol bombs and two improvised explosive devices were thrown at officers during the sixth consecutive night of violence.
"We already have had police officers who have suffered injuries, thankfully not too serious. Many of them were actually back at work serving the community," said Mr Hamilton.
"But if this continues, it's only a matter of time before a police officer or some child or young person involved in this violence gets very seriously injured, or worse."
'Not in our name'
Hundreds of people gathered on Friday night for a rally against the violence.
The crowd came together at the top of Fahan Street to say "not in our name".
Those in attendance included Sinn Féin president Mary Lou MacDonald, Sinn Féin vice president Michelle O'Neill and SDLP leader Colum Eastwood, along with other party members.
The Bishop of Derry Donal McKeown also spoke at the podium along with Jim Roddy, Derry's city centre manager.
Bishop McKeown said: "You cannot claim to love your country and at the same time cause pain and destruction to the people who live there.
"Nothing can be built with petrol bombs or stones thrown in anger."
Community workers were also there along with hundreds of residents from different parts of the city.
The police said children as young as eight were involved and accused dissident republicans of being behind the attacks.
The PSNI also said the six nights of violence in the city had seen two separate attempts to murder officers.
A Twelfth of July parade passed off peacefully in Derry on Thursday, although police in riot gear separated a group of nationalists from the return parade as it made its way through the Diamond in the city centre.
Attacks began early in the evening and continued into the early hours of Friday morning.
Petrol bombs were thrown towards police Land Rovers and at officers on the ground.
Makeshift barricades were built and set alight on the Lecky Road flyover and on Fahan Street, while police closed sections of the walls overlooking the Bogside.
On Friday, Mr Hamilton said several officers and a public order inquiry team were viewing footage of the disorder and he expected more arrests.
Senior PSNI officials said it was "a miracle" officers were not injured.
"I am saddened, disheartened and frustrated that yet again police officers committed to keeping this local community safe have been attacked by a small number of people within it," said Supt Gordon McCalmont.
"While we have seen many young people involved in these attacks over the last number of nights, it is clear that this is being orchestrated by a more sinister, adult, violent dissident republican element.
"This cannot continue. This must end now."
The Walls are now closed heee in Derry after some petrol bombs were thrown from the Bogside pic.twitter.com/lwFNotusLW— Kevin Sharkey (@tv_KevinSharkey) July 12, 2018
Secretary of State Karen Bradley said the "intolerable violence by a small minority" was completely unacceptable, as she voiced her support for police efforts to bring an end to the unrest.
DUP leader and former first minister Arlene Foster said: "Really disturbing scenes last night in Londonderry. Someone will be killed if this continues."
The SDLP leader said the violence was only succeeding in "terrifying people in their own homes".
"These people who are supposedly republicans, who are supposedly trying to get a United Ireland are destroying their area," said Mr Eastwood.
"Now, I don't know how many times we have to learn this lesson that this does not work.
"It's not only morally reprehensible, it's strategically stupid."
Sinn Féin MP Elisha McCallion said "sinister elements" were spurring on and controlling the violence.
"People in the Bogside were afraid for their lives last night," she said.
"This has gone beyond recreational rioting, this is disgraceful and those involved in it and indeed those behind the scenes stirring up this type of activity, need to catch themselves on."
We regret to say that we are experiencing further disorder in the area of the Bogside with a number of petrol bombs already thrown at police. There is a large crowd gathering. It is clear that there are young children involved. Parents - make sure your children aren’t involved.— PSNI DC&S District (@PSNIDCSDistrict) July 12, 2018
On Tuesday, police said dissident republicans were responsible for automatic gunfire directed at officers.
On Thursday, two viable devices were thrown at officers on Derry's walls, police said.
Supt McCalmont described that attack as a "reckless act" that showed "a blatant disregard for life".