A man was brazenly stabbed to death as a nearby Garda requested assistance in monitoring his interaction with another man on Dublin's O'Connell Street.
The victim and another man had already caught the attention of a Garda on duty in the area who requested they be monitored by Garda CCTV but an exchange between the men rapidly escalated into a row.
By the time the Garda reached them to intervene, one of the men had received a serious stab wound.
The Garda administered first aid and paramedics were quickly on the scene but the man died a short time later at the Mater Hospital.
The incident happened at 1.40am this morning towards the Parnell Street end of O'Connell Street, a short distance from the GPO and the Spire.
A suspect, a man in his late 20s, was arrested within 15 minutes outside the Pro Cathedral Marlborough Street area but his questioning was suspended during the night to allow him to be brought to hospital for treatment to wounds to his hands.
He remains in custody this evening.
The identity of the victim was not known and there was speculation that he may have been homeless.
Brian McLoughlin of Inner City Helping Homeless said he had heard this speculation but the organisation was unable to confirm this.
"We'd normally know from our own workers or our service users if something happened to someone on the streets because a lot of them know each other and keep an eye out for each other but we haven't heard anything about the identity of this man," he said.
A large section of O'Connell Street and part of adjoining Cathal Brugha Street were sealed for forensic examination, shutting shops and closing the roads to traffic until mid-afternoon.
Commuters experienced major disruption as the Luas could not enter the main thoroughfare and Dublin Bus and Bus Éireann services had to be diverted.
A Garda statement acknowledged the difficulties caused to transport and business but said: "a full and thorough investigation into this fatal incident must be carried out".
Gardaí are examining security cameras for footage they believe captured the incident but they are also appealing for anyone who has phone or dashcam images to allow detectives to view them.
They are seeking help in piecing together the events that led to the attack and want to talk to anyone who was in the area between 1am and 2am and saw anything of note.
Supt Paul Costello said: "Any member of the public who may have taken any mobile phone footage of this incident or any mobile phone footage or dashcam footage in these streets at that time are asked to contact An Garda Síochána."
Knife crime has declined in the last few years but the problem of knives on the streets has not.
In March, Gardaí revealed that seizures of knives from people carrying them in public had jumped by 66% in two years.