A major training exercise involving a simulated explosion on a nuclear weapons convoy is being staged.
Emergency workers will be testing their procedures
The Ministry of Defence (MoD) exercise in Edinburgh, entitled Senator 05, is designed to test Scotland's response to a serious disaster.
It involves nearly 1,000 people and is based on a scenario where a plane engine falls onto a nuclear convoy.
Emergency services are dealing with simulated explosions, smoke and students acting as casualties.
The event is taking place at Dreghorn Barracks in the capital.
Authorities stressed that the exercise has been planned for two years and has nothing to do with the threat of terrorism or the recent bombings in London.
The mock disaster began with a nuclear weapons convoy passing Edinburgh Airport. This was struck by an engine falling from an aircraft which had just taken off.
In the exercise, a road tanker then lost control and crashed into the same vehicle, creating a massive disaster zone.
Rear Admiral Nick Harris, of the Royal Navy, said a run-through of this kind and on this scale would normally happen about once a year in the UK.
Safety and security
He said: "This is nothing whatsoever to do with terrorism, but I think it's important to note that any kind of emergency response exercise means that people rehearse skills that are transferable."
The rear admiral added: "The MoD has an exemplary nuclear safety record and continues to place the highest importance on the safety and security of nuclear weapons."
Assistant Chief Constable Tom Halpin, of Lothian and Borders Police, said more than 100 officers are involved in a co-ordinating role between all the agencies.
He said: "The scenario we're exercising here truly can be described as extreme and I really would never foresee us experiencing that.
"We need to test the boundaries of our capability and it's only by using such an extreme example that we're able to test that."
Bill Ness, head of emergency planning at the City of Edinburgh Council, said this exercise is the largest to be carried out in the city in the past 10 years.
The drill spans three days, with press officers, council workers and staff at the Scottish Executive among those also getting the chance to play out the roles they would take in any similar emergency.
But environmental campaigners said the exercise would be unnecessary if nuclear weapons were scrapped.
Anti-nuclear group NukeWatch Scotland said it did not make them feel safer or happier about weapons of mass destruction travelling on Scotland's roads.
Green MSP Chris Ballance, who speaks on nuclear issues for the party, added: "Our continued possession of nuclear weapons threatens all Scotland.
"Britain must honour our commitments under the Nuclear Non-Proliferation Treaty and take real steps to disarm.
"I am appalled that the MoD sends nuclear warheads through major population centres in Scotland."