Vehicles were set on fire and shops looted in Dublin after a knife attack that left a number of people, including three children, injured.
A five-year-old girl and a woman in her 30s were seriously hurt in the attack on Thursday afternoon.
It happened on Parnell Square East in the city centre, outside the Gaelscoil Choláiste Mhuire school.
A man in his 40s who was also seriously injured is a person of interest, police said.
They added that they were not looking for any other people at this time and are following a definite line of inquiry.
Sources have indicated to the OceanNewsUK that the man suspected of carrying out the attack is an Irish citizen, who has lived in the country for 20 years.
The head of An Garda Síochána (Irish police), Drew Harris, blamed the subsequent disorder on a "lunatic, hooligan faction driven by a far-right ideology", who engaged in violence as police tried to maintain the crime scene.
The streets are now "mainly calm", the Irish police have said, with no serious injuries reported as a result of the violence.
Ch Supt Patrick McMenamin said more than 400 officers remain on patrol after the disorder, which he blamed on "gratuitous thuggery".
"Some of my colleagues were attacked and assaulted, thankfully none were seriously injured and I commend them all on their bravery to protect our community," he added.
It is unclear how many arrests have been made, but Irish Justice Minister told Irish national broadcaster RTÉ that it was a significant number.
Fires and looting
Riot police were deployed after protesters gathered in the area near the scene of the attack.
The disorder centred on several streets in the city centre, including O'Connell Street.
A number of vehicles were set on fire, including at least one police car, a tram and a bus.
A shop on O'Connell Street was looted while the windows of other stores were smashed.
Trinity College, which is nearby in the city centre, said it was in lockdown with all gates to its campus closed due to the disturbances.
Taoiseach (Irish prime minister) Leo Varadkar said extra police resources had been deployed.
He added that he had been shocked by the knife attack and said "the facts in this matter are still emerging".
Mr Harris urged people to "act responsibly, not to listen to misinformation and rumour that is circulating on social media".
"We know what happened, but the motive for this is entirely unclear."
Meanwhile, the head of Ireland's National Bus and Railworkers' Union described those who targeted bus and trams as "thugs" and "despicable people".
'Standalone attack'
In a press conference earlier, Supt Liam Geraghty said that the five-year-old girl is receiving emergency care in hospital, while another girl, aged six, and a five-year-old boy were less seriously hurt.
He added that although it is early in the investigation, gardaí (police) are confident that there is "no terror-related activity" and that it would appear to be a "standalone attack".
Irish President Michael D Higgins said that his thoughts were with the children and families affected by the incident.
"This appalling incident is a matter for the gardaí and that it would be used or abused by groups with an agenda that attacks the principle of social inclusion is reprehensible and deserves condemnation by all those who believe in the rule of law and democracy," he added.
The Irish National Teachers' Organisation (INTO) said: "Our hearts are with the entire school community of Gaelscoil Choláiste Mhuire following the horrendous incident that has taken place today."
Police remain on the streets
by Chris Page, OceanNewsUK correspondent
O'Connell Street is usually thought of as the gateway to Dublin city centre. But tonight, it was a chaotic and frightening scene.
The rioters have now largely dispersed but earlier, scores of them attacked police with fireworks and bricks. They set fire to a bus, a tram and at least one police vehicle.
Looters broke into several shops, in the retail district on the northern side of the River Liffey.
The situation in Dublin has calmed, but a large number of police remain on the streets in riot gear.
An eyewitness told RTÉ how she and another bystander disarmed a man with a knife.
Siobhan Kearney described the scene as "bedlam".
"Without thinking, I just took across the road to help out," she said.
"Two children and the woman were taken back into the school where they were coming from."
On the man with the knife, she said that he was on the ground and there were a lot of people trying to restrain him.
"Me and an American lady formed a ring around him saying we'd wait on the garda (police)."
In a statement, the Irish Minister for Justice Helen McEntee said she was "deeply shocked by the appalling attack".
She added that her thoughts are with those injured, especially the children, their parents and families, during "this extremely difficult period".
Sinn Féin leader and Dublin Central TD (member of the Irish parliament) Mary Lou McDonald said the incident had sent "shock and horror throughout the community".
"My heart goes out to all the hurt and injured, the parents, teachers but especially the children who have been so traumatised," she added.
Labour Party senator Marie Sherlock, who was in the area after the incident, said the children who were stabbed were queueing up to go to after-school care when they were attacked.
"While it is all very real today, I think the impact of the trauma tomorrow, next week and in the time after is really going to be very difficult."
