Gunmen who dressed in women's clothes and SWAT team uniforms attacked a boxing event armed with "AK-47-style" rifles and handguns, leaving one man dead and two more injured.
A major manhunt is under way to find four attackers who burst into the Regency Hotel in Dublin, where up to 300 fans including children had gathered for the weigh-in on Friday ahead of a fight between Jamie Kavanagh and Antonio Jao Bento.
Bystanders ran for their lives as the gang, one of whom was disguised as a "woman and wearing an auburn wig" and the others, dressed as "crime drama" police officers, opened fire in the lobby.
The victims, aged in their 20s and 30s were taken to hospital where a man in his 30s, who was "riddled" with bullets, later died.
Gardai Chief Superintendent Barry O'Brien said it was a "particularly nasty incident" that had involved "severe weapons".
He said: "Two individuals were wearing what was described as police-type uniforms. In that regard it was described as a SWAT uniform with metal helmets, similar to what you see on crime dramas."
Bystanders in the lobby of the Regency Hotel on the Swords Road, around two miles north of Dublin city centre, described cowering for their lives as the attackers fired multiple rounds at the men.
Kevin McAnena, a sports reporter who was covering the event, said one of two gunmen pointed a "big, massive" gun at him while he sought cover behind the reception desk.
Mr McAnena said: "I was shouting even before he came over, 'don't shoot, don't shoot' ... I've never felt terror like it."
Mr McAnena said he saw a gunman firing into a victim's leg.
"The man who is now dead - he was no more than six foot from me.
"The gunshots I heard after I jumped over the receptionist's desk were probably the gunshots that finished him off.
"That same gunman looked over at me before pointing the gun at me, telling me, I think he said 'get up' or 'go away', or something like that."
Mr McAnena heard two shots outside the hotel and at least three more inside.
"They were big, massive guns - gardai have told me since they were AK-47 rifles," he said.
"It was just so surreal, it happened so quickly."
In dramatic amateur video of the attack, rapid gunfire can be heard erupting in the lobby just seconds after the men left the stage in a function room at around 2.30pm.
One child is heard crying: "Daddy, help me. Daddy, what was that?"
As chairs are tossed aside and people dive for cover, one petrified woman is heard saying "What the f*** is going on?"
Some spectators fled from the room while heavy rock music continued to play through speakers.
Witness Mel Christle, president of the Boxing Union of Ireland, was standing on a podium where the last of the boxers weighing in had just stood down when the gunfire erupted outside the room.
He said: "There was an incredible noise out there (in the lobby) and that is where I saw a body, a corpse, lying literally at the edge of the reception desk."
It is believed a number of gangland figures were expected to be attending the weight-in.
Speculation has centred on a feud between Dublin gangsters and an Irish criminal outfit based in the south of Spain.
The boxing chief was adamant the attackers were not targeting the boxers gathered for the weigh-in, but other individuals who were hit in the hotel reception.
The gunmen fled the attack in a van, which was later found set alight nearby.
Mr Christle said one of the attackers was dressed as a woman and aged in his early 20s, a detail later confirmed by police.
The crowds had gathered to watch a weigh-in for a World Boxing Organisation (WBO) lightweight title fight between Jamie Kavanagh and Antonio Jao Bento before their Clash of the Clans bout, which was scheduled to take place on Saturday evening,
Kavanagh later tweeted: "Anyone asking I'm OK! Thanks you for asking. I was lucky today is all I can say....."
Box Nation, the television channel scheduled to broadcast the fight, said the show had been cancelled due to the shooting.