A counter terrorism exercise is currently being carried out by An Garda Síochána in Dublin city centre.
There are three scenarios being played out including a car crash and individuals being stabbed by the attacker.
The incident occurred at a train station in Spencer Dock where two men drove a van into a pedestrian. They then travelled on foot, armed with a knife and gun, into the station.
Gardaí, including an armed unit, arrived at the scene shortly after the men entered the station.
The two men attacked more people in the station until one of them was captured by gardaí after hand grenades were detonated.
The other man made into on to a train with a gun taking passengers hostages. There is currently a negotiation taking place between him and gardaí.
According to an Garda Síochána the focus of the exercise was to "simulate a terrorist incident which would prompt appropriate responses from local Garda strategic and operational command personnel, Gardaí from the Dublin Metropolitan Region, ERU (Emergency Response Unit), ASU (Armed Support Unit), Garda Air Support unit and staff from Irish Rail."
It has been planned since last April and Garda Commissioner Noirín O'Sullivan is on site observing.
Superintendent Sean Ward says it’s been a long time in the planning;
He said: "It's been planned since last April, it's an exercise that's been lead on from other exercises that have been taking place with the ambulance service, with the fire service and with the military over the last two years.
"It's an ongoing process in terms of exercises, and there will be further exercises later on in the year as well, but this is a live exercise."