Police forces in England and Scotland are to take part in a major counter terrorism exercise to test the response of emergency services to a cross-border incident.
The three-day "live-play scenario", which will involve locations around Edinburgh, the Lothians and Northumbria from today, is not in response to any specific threat but is the result of over a year of planning.
Officers will inform people in local residential and business areas close to the exercise so as not to raise alarm through noise or a large presence of emergency services.
Named Exercise Border Reiver, it will not affect day-to-day policing or emergency responses, which will continue as normal throughout the exercise, Police Scotland said.
Other bodies involved in the exercise include Northumbria Police, the Scottish Fire and Rescue Service, Scottish Ambulance Service, North East Ambulance Service, Northumberland and Tyne and Wear Fire and Rescue services, Transport Scotland, NHS Scotland, NHS England, the Ministry of Defence and officials from the Scottish and UK Governments.
Assistant Chief Constable Darren Best, of Northumbria Police, said: "This exercise has been organised to test the response of the emergency services and other partner agencies to a cross-border incident.
"It is in no way linked to a specific or increased threat in our region.
"We regularly carry out exercises to test contingency plans and the co-ordination of the response to a major incident to ensure we are prepared to keep our communities safe.
"We want to reassure people the exercise will not affect day-to-day policing and emergency service response, which will continue as usual throughout the exercise."
Police Scotland Assistant Chief Constable Bernard Higgins said: "Police Scotland and other police forces throughout Britain regularly assess and exercise their counter terrorism resources and capabilities to ensure we can respond appropriately to any incidents, which may arise.
"Exercise Border Reiver is the culmination of months of planning with partner agencies from across the UK and the scenario that will unfold over the three days is a realistic, challenging and complex set of circumstances that could arise during a terrorist incident.
"In addition to uniformed officers, a range of specialist Police Scotland resources will be in play for the exercise, and this will result in some significant activity within the public domain.
"Please rest assured this all forms part of the live-play scenario and there is no risk to the public.
"The public should be reassured that we are constantly testing and exercising our plans along with our partners.
"Police Scotland is committed to ensuring that Scotland is capable of responding to the terror threat that faces the UK."