Counter-terrorism training exercise held in south Wales
A counter-terrorism training exercise is being led by South Wales and Dyfed-Powys Police to test the response of emergency services to such an attack.
Scenarios are based on "prevailing" threats following attacks in France and Belgium, said the officer in charge.
The two-day exercise is being held at locations including Ffos Las racecourse near Kidwelly, Carmarthenshire.
People have been told not to be concerned by noise of blank gun fire and simulated explosions in the area.
As well as armed police and other 999 crews responding to the initial callout from a "no notice terrorist attack", senior officers are managing the incident and liaising with officials from the Welsh Government and UK Government to test coordination at a national level.
Events are also taking place in Bridgend in Bridgend county and at Llantrisant Common in Rhondda Cynon Taff.
South Wales Police Assistant Chief Constable Jon Stratford, who is leading the operation, said the UK Government's emergency response committee, Cobra, involving ministers and intelligence officers, would also participate.
He added: "This isn't being hosted in Wales because we think there is a threat to Wales. This is part of a rotating exercise programme.
"Clearly, we model our scenarios upon the prevailing threat and so Paris and Belgium do feature heavily in these scenarios.
"What the exercise will test is not just the ability of frontline police officers, paramedics, fire officers to actually respond to the scene, but we will also be standing up the command and control measures that work across the force areas.
"I urge the public not to be alarmed... it is all part of the exercise. We need to make these exercises as realistic as possible."