The Police Service of Northern Ireland is seeking the public's assistance in identifying individuals involved in disorder.
Some 63 police officers were injured in what has been called a "week of shame" in Northern Ireland.
Disorder started in Ballymena on Monday after an alleged sexual assault of a girl in the Co Antrim town at the weekend, and continued throughout the week.
Assistant Chief Constable Ryan Henderson said the force is "actively taking steps to find" those involved in the rioting.
At a press conference, the PSNI released images of four "persons of interest" it wants to identify and interview in connection with its investigations.
"In releasing these images, I am asking the wider community to step forward and help us to identify these people.
"It is in all of our interests and in the interests of justice that those responsible are dealt with and I would urge anyone who may have information to bring it forward."
Mr Henderson said 17 people have been arrested in connection with the disorder to date, adding that 13 of those have been charged with criminal offences
Weekend preparations
There will be "a large policing presence across Northern Ireland" over the weekend, a senior officer has said.
Mr Henderson said the mobilisation, which would include officers from Scotland, was "to reassure our communities and protect our streets".
He said he hoped the disorder was over but added: "I have prepared, and my colleagues have prepared for the weekend ahead another policing operation scaled up in anticipation to be able to deal with things that may occur over the weekend."
A number of homes were targeted during the disorder, and the Northern Ireland Housing Executive said about 50 households have received assistance across the week, and 14 families provided with emergency accommodation.
While disorder in Ballymena raged across Monday, Tuesday, and Wednesday nights, it appeared to have abated yesterday. However, there was still disorder in Portadown.
Pressed on whether loyalist paramilitaries were present in Portadown, Mr Henderson said: "We're still very much in the throes of trying to identify who was involved last night in the disorder."
He added: "But we absolutely did see in Portadown last night, people who were directing young people and directing others back and forwards to try and get around police lines, find weak points, throw weaponry, etc.
"So we certainly saw more coordination in the activity last night than we had seen in previous days. As to who was doing that coordinating, I'm not in a position to say yet."
There have also been incidents in other towns, including the burning of Larne Leisure Centre on Wednesday and an arson attack on a house in Coleraine in the early hours of Friday from which a man and woman, and four young children escaped.
It is being treated by police as arson with a racially-motivated hate element.
Twenty-two police officers were injured in overnight disorder in Portadown after they came under sustained attack with heavy masonry, fireworks and beer kegs in the Co Armagh town on Thursday.
A woman in her 50s and a man in his 30s were arrested on suspicion of riotous behaviour and other offences in connection with the disorder in Portadown.
There was also sporadic disorder elsewhere in Northern Ireland last night, following mainly peaceful protests, including in the Templemore Avenue area of east Belfast, where bricks were thrown through the windows of two houses in Avoniel Road in what police have called a racially motivated attack.
A small fire at the Manse Road roundabout in Newtownabbey was also reported, and anti-immigration hate graffiti is being investigated in Newtownards.
Meanwhile, a house fire in the Mount Street area of Coleraine which led to the evacuation of a family with three young children is being treated as deliberate and a racially motivated hate crime.
Additional police officers from Scotland will support the Police Service of Northern Ireland after a mutual aid request.
'Totally deplorable'
The Police Federation, which represents rank and file officers, said that since violence erupted in Ballymena on Monday, 63 officers have been injured.
Federation chairman Liam Kelly said it has been "a week of shame with appalling levels of unrest in towns and cities".
"Burning people out of their homes, attacking a leisure centre, and the specific targeting of individuals and property fuelled by overt racism and prejudice, is totally deplorable," he said.
"Yet again, our overstretched police officers have also been attacked with petrol bombs, bricks and fireworks as they sought to keep people safe and maintain law and order.
"They have held the line with great courage and professionalism and are owed a debt of gratitude by this community.
"They went to the aid of vulnerable people, have prevented further savage attacks and have undoubtedly saved lives."
Watch: PSNI coming for 'bigots' following Ballymena unrest
Mr Kelly called for an end to the violence.
"This has been a week of shame with hate-filled rioters turning our streets into battle zones, targeting people and property simply because of their perceived nationality or the colour of their skin," he said.
"We will continue to police our society without fear or favour, and all persons, irrespective of their background, race, colour or creed, who are engaged in any criminality, should expect to be the subject of our criminal justice processes.
"We cannot have sections of our society blatantly disregarding the law and bringing their own twisted version of justice to bear.
"As we head into the weekend, I would once again appeal for calm. Bringing violence, fear and uncertainty to our streets is totally unacceptable and must stop.
"I would ask all parents to speak with their children, know where they are going in the evenings, and actively encourage them to be civically responsible."
'Bigots and racists'
Police Service of Northern Ireland Chief Constable Jon Boutcher warned that his officers would be coming after the "bigots and racists" behind the disorder.
He also said that the young girl who was the victim of an alleged sexual assault in Ballymena at the weekend had been "further traumatised" by the rioting across the week.
In a statement on Friday, Assistant Chief Constable Ryan Henderson said police had come under sustained attack in Portadown.
"Officers deployed a variety of public order tactics to restore order to the town as quickly as possible," he said.
"Twenty-two officers were injured and I am so grateful to them for their bravery and selfless dedication in protecting the community of Portadown from this criminality.
"We saw calm in Ballymena and I hope the powerful words of the family who asked for calm during this really difficult time were heard and continue to be heard over the coming days."
He said that while two arrests have been made so far in terms of the disorder in Portadown, more will follow.
"I repeat, once more, our appeal for calm across Northern Ireland in the coming days," he said.
