A large scale exercise to test how the emergency services will need to react if a major incident strikes during the London 2012 Games will be staged this week.
Lessons learned from the July 7 bombings will feed in to the two-day live exercise - dubbed Forward Defensive - in central London.
The disused Aldwych underground station, near London's Royal Courts of Justice, will be the scene of a mock incident on the tube network on February 22. Passers-by will be able to spot people being evacuated as part of the mid-morning test.
Other less visible security work will be carried out through to the following day.
The two-day event will be staged as if it is August 8 an 9, which will be two very busy days in London during the Games.
It will involve everyone from "constable to Cobra," the top level Government committee which sits during national emergencies and times of crisis, the National Olympic Security Coordinator, Assistant Commissioner Chris Allison said.
Emergency responses dealing with casualties and the crime scenes will be tested along the co-ordination and communication systems across a range of organisations that have been brought together for the Games.
It will be a chance to see how decision-making on the ground works in relation to the complex command and security planning which has been worked out on paper and in mock table-top exercises.
Mr Allison said: "This is part of a national exercise.
"The majority of stuff that the public will see, because clearly they will not be underground, will be the people coming out of the Tube system afterwards.
"It is for us testing our first responders capabilities and what we have learned to make sure that we have got that in place from 7/7 (the July 7 2005 bombings).
"Then there is the follow-up across all these commands and communications nodes who normally do not work together.
"Here we are doing it in a live exercise. It is testing communications right from the very bottom from the constable or fire officer who is responding right the way up to Cobra."
Crime and Security minister James Brokenshire said: "The Home Secretary and I will be involved.
"There will be other senior ministers who will be engaged in the exercise. It will, in many ways depend on how the exercise itself flows and that is part of the live play process. The exercise takes place, events happen and it gets escalated to Cobra and depending on the nature of that activity that will determine who actually gets involved in it.
"I can certainly assure you that senior members of government will be actively involved."
The exercise will test the knock-on effects to other 2012 venues in places such as Dorset and Surrey and how command and safety procedures will have to be stepped up.
"Clearly anything that may happen in London, we would also have to consider any impact on other Olympic venues at that time," Mr Allison said.
Communications between key strategic decision-makers will also come under the spotlight.
This includes the police National Olympic Co-ordination Centre at Scotland Yard, the transport co-ordination centre and the London 2012 organisers' main operation centre. A number of Cobra meetings will be called.
"It is a good test of what it is going to be like in Games-time," according to Mr Allison .
Use of the military has not been ruled out.
Mr Allison said: "Clearly there is the option - it just depends on how the scenario works out."
Forward Defensive is part of the "mature" security plans which have been designed for the Games, according to Mr Brokenshire.
Assistant Commissioner Specialist Operations Cressida Dick, who will be the National Director of Counter Terrorist Policing during the Games, described it as "one of the extreme scenarios that we could face".
She said: "This exercise and its scenario is not as a specific result of intelligence.
"The scenario has been planned for many many months. We will be doing our very best to prevent such an attack but it would only be right that we test our response to such an attack.
"This is one of several tests and exercises that we have been doing and will be doing over the coming months.
"This is a large-scale exercise for us."
She suggested that the Olympic safety and security planning is "building on a very strong platform" describing Britain's counter-terrorism policing as "very capable, flexible, very resilient".
Officers from different forces are trained to the same level, understand each others businesses and can work co-operatively, she said.
The current terror threat level is substantial from international and Northern Ireland sources.
Planning has been carried out for a severe threat level during the Games with a contingency that it could be stepped up to critical if needed.
Serious crime, protests and natural hazards are other risks to the Games.