Irish police said five people, including three young children, had been taken to hospital on Thursday following what local media said was a stabbing in Dublin city centre.
Prime Minister Leo Varadkar said police had detained a suspect.
National broadcaster RTE quoted a witness on the scene as saying she saw a man in a "stabbing motion" from across the road and that when she ran closer, she said that two children had been stabbed as far as she could tell.
Anthony Boyle, 31, an IT consultant who lives on the road and was passing by told Reuters, "I saw a child on the ground, a little girl."
"There was complete and utter pandemonium, women wailing, men screaming and crying."
He added that he saw a man lying on his side but did not see the events before.
One girl has sustained serious injuries and the other two children are being treated for less serious injuries, police said. An adult female is also being treated for serious injuries and an adult male has less serious injuries, police added.
Local media reported that the people were stabbed on Dublin's Parnell Square, which is next to Dublin city's main thoroughfare of O'Connell Street. There were around 20 officers on the scene that police said remained sealed off.
"We are all shocked by the incident which has taken place in Parnell Square," Varadkar said in a statement.
The Irish Times reported that the detained suspect had sustained wounds believed to be self-inflicted. It said early indications suggested a man tried to attack a number of young people and that passers-by intervened.
The motive for the attack, which occurred close to a school on the street, has not been established, the Irish Times added.
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