Three young children and a woman in her 30s were injured near a school in Dublin on Thursday, the police said, in what the country’s justice minister described as “an appalling attack.”

A suspect in the case was in custody, according to a spokeswoman for the Garda Síochána, the Irish police force.

The police said a knife was used in the attack, and that an adult woman in her 30s and a 5-year-old girl had sustained serious wounds, while the other two children, a 5-year-old boy and a 6-year-old girl, were being treated for less serious injuries. They said the episode, which they described as “a serious public order incident,” took place at Parnell Square East, in central Dublin, shortly after 1:30 p.m.

Liam Geraghty, a spokesman for the police, said at a news conference after the incident that according to preliminary information the knife attack was a standalone event and not an act of terrorism. He added that investigators were following a definite line of inquiry and were not looking for other suspects.

The police force said in a statement later that it was keeping “an open mind at this early stage of the investigation.”

“An attack on children is an act of absolute cowardice and depravity,” Gary Gannon, a member of Parliament for the area, wrote in a statement.

On Thursday evening, violent scenes broke out on the streets of Dublin after a group of rioters attacked police vehicles and set fires.

The police blamed a far-right faction and asked people to leave the streets.

Drew Harris, the Garda police commissioner, said the riots were “disgraceful,” and suggested they were driven by misinformation about the knife attack that was being spread online for “malevolent purposes.”

The suspect in custody was a man in his late 40s or early 50s, who was also injured, the police said.

The five injured people were taken to hospitals in the Dublin region, according to the police, who said they were in contact with all the children’s parents.

In a statement, the Irish justice minister, Helen McEntee, called the knife attack “an appalling crime” but said “the scenes we are witnessing this evening in our city center cannot and will not be tolerated.”

“A thuggish and manipulative element must not be allowed use an appalling tragedy to wreak havoc,” she said, appealing for calm and adding that she would meet with the police commissioner to discuss the policing plan for the city.