Ten police officers were injured after petrol bombs, fireworks and heavy masonry were thrown during disorder in Derry.
Police said they were liaising with community representatives to prevent further escalation after disorder erupted in Nailors Row.
The Police Service of Northern Ireland (PSNI) appealed to parents and guardians to take their children home.
They warned that the disorder was presenting a “serious risk” to the safety of residents in the area.
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A number of tactics were deployed in an attempt to de-escalate the situation, including use of the Public Order Dog Unit.
Superintendent William Calderwood said: “The scenes we witnessed were disgusting, and I want to commend our officers for their professionalism. These were difficult conditions for officers from across numerous departments. To see our officers injured as a result of this violence is appalling. It is completely unacceptable.”
The PSNI said one arrest had been made in connection with the incident but a “robust investigation is under way”.
Separately, a petrol bomb was thrown at a mosque in Co Down in the early hours of Saturday, in an incident described by police as racially motivated.
The mosque in Newtownards was attacked at about 1am, with graffiti sprayed on the front door and walls of the building on Greenwell Street.
Police said the petrol bomb did not ignite.
Chief inspector Keith Hutchinson said: “This is being treated as a racially motivated hate crime, and I want to send a strong message to those who carried this out that this type of activity will not be tolerated and any reports of hate crime are taken very seriously.
“I am appealing to anyone with any information or footage that could assist with inquiries to come forward and help to identify those responsible and put them before the courts.”
Police said several protests in Belfast city passed without incident on Friday evening, but officers were investigating a number of overnight reports of damage to property and vehicles.
Cars were set alight on Tavanagh Street and Sandhurst Gardens and police said the incidents were being treated as racially motivated hate crimes.
The rear door of a restaurant in Ormeau Road was also kicked, with racial slurs shouted at the workers inside.
Five people were arrested on Friday.
A 22-year-old man was arrested after Friday’s protests in Belfast accused of throwing a bottle at a police vehicle.
He will appear in court on September 6th charged with offences including disorderly behaviour and criminal damage.
A man (51) arrested after the city centre protest has also been charged with disorderly behaviour and will appear in court on Saturday.
Meanwhile, two boys (14) were arrested in Derry. The arrests came after a number of petrol bombs were found by police in the vicinity of Spencer Road.
One teenager was charged with possession of petrol bombs in suspicious circumstances, while the other teenager was charged with possessing articles for use with petrol bombs.
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They appeared at a special sitting of Dungannon Magistrates’ Court on Saturday.
A man (37) who was arrested in Portrush on suspicion of intentionally encouraging riot in relation to online activity remains in police custody.
Police also arrested a man (48) in the Belfast area on suspicion of intentionally encouraging riot. He remains in police custody.
Officers in south Belfast investigating disorder at Broadway Roundabout on July 15th and 16th arrested a boy (14) on suspicion of riot and possession of an offensive weapon. He is in custody assisting officers with their inquiries.
Temporary assistant chief constable Melanie Jones said: “We continue to investigate all reports made to us in relation to the recent disorder on our streets and we will be making further arrests.
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“The outpouring of support from our communities for their neighbours who have chosen to make Northern Ireland their home has been heartening, and more accurately reflects the views of most of the citizens of Northern Ireland.
“We will continue to have a significant visible policing operation in place across Northern Ireland over the weekend to ensure our communities are kept safe.”
It comes after thousands of people took to the streets of Belfast on Saturday in an antiracism demonstration. – PA