Hundreds of police, firefighters and ambulance crews raced to the scene of the incident, a former mental hospital at Dechmont, West Lothian.
Donning gas masks and biohazard suits, they were faced with up to 250 victims requiring emergency treatment including decontamination.
Fortunately, the terrifying incident was not real, but was a test designed to ensure emergency services in the UK are prepared to cope with a genuine incident.
The aim of yesterday's exercise, which involved emergency personnel from across the UK, was to determine how they handled the aftermath of a radiological incident in a busy built-up area.
Those taking part were told that the scenario was the explosion of a dirty bomb in Edinburgh city centre. The main task for was to treat the wounded and decontaminate survivors. The exercise involved around 700 people, including more than 200 police officers, more than 100 fire and rescue services personnel, and 60 members of the Scottish Ambulance Service.
There were also 250 members of the public drafted in to play the part of victims, complete with gruesome realistic "wounds" created by make-up artists.
Justice secretary Kenny MacAskill said: "Exercise Green Gate is giving the police, fire and ambulance services the opportunity to work together to rehearse their roles in the event of an incident requiring the decontamination of a large number of people.
"It is important that we ensure that Scotland is prepared to deal with the consequences of any emergency."
The volunteers were mainly medical students from Glasgow University, allowing them to better mimic symptoms of radiation poisoning. They were joined by SAS college volunteers, as well as people from Amputees in Action, Casualty Bureau, British Red Cross, and Forth Valley College students.
West Lothian Council contacted local residents in writing ahead of the event, advising them to stay away from the area and not to be concerned by the high volume of emergency vehicles.
Chief Superintendent Graeme Dobbie of Lothian and Borders Police said: "This exercise is particularly beneficial as it allows us to test our plans in a practical way while working alongside colleagues in the other emergency services."