Garda Commissioner Drew Harris said it appears as though members of the public intervened in a stabbing incident in Dublin, in which two adults and three children were injured.
He was due to meet with the Policing Authority in Waterford, but was interrupted by the news from the capital.
Mr Harris said: "It would appear that following the incident itself, and in part of the incident itself, members of the public who were passing by intervened and in effect grappled with this individual... and that allowed gardaí on their arrival to arrest this individual."
Gardaí confirmed a five-year-old girl sustained serious injuries and is currently receiving emergency medical treatment in CHI Temple Street.
A five-year-old boy and a six-year-old girl sustained less serious injuries and were taken to CHI Crumlin for treatment. The boy has since been discharged.
A woman aged in her 30s is receiving treatment for serious injuries at the Mater Hospital.
Gardaí added that man in his 50s, who is a person of interest in the investigation, also sustained serious injuries and is being treated at another hospital in the region.
There are no Green Line Luas services between St Stephen's Green and Dominick Street this evening because of the incident on Parnell Square.
Dublin Bus meanwhile says diversions are in place on Southbound Routes that usually pass the Square.
They will go by Denmark Street, Gardiner Street, Cathal Brugha Street and O'Connell Street before rejoining the ordinary route.
Bus Éireann services 103, 109, 109Z, 111 and NX will be re-directed by Gardiner Street until further notice.
