Passengers have been staggering from a tube station in central London - as part of a large scale security drill for the London 2012 Games.
Emergency services swarmed around the disused Aldwych station to deal with 'casualties' in the mock terror attack on the transport network.
Office workers and passers-by could see the evacuation, which marked the beginning of a two-day security exercise dubbed Forward Defensive.
The test - the biggest to date - is being staged as if it were August 8 and 9, which are expected to be two of the busiest days in London during the Olympics.
It involves around 2,500 people from police, fire and ambulance staff to Olympic chiefs and Cobra, the government committee which sits during national emergencies and crises.
Lessons learned from the July 7 bombings in which 52 people were killed were being fed into the trial, most of which is taking place behind closed doors.
Last year, an inquest into the attacks - in 2005 - made a series of recommendations, including training in emergency situations for frontline staff.
Howard Collins, London Underground's chief operating officer, said: "We have certainly picked up a number of recommendations.
"For example the response for our vehicles are now escorted under blue light conditions to get through London quickly.
"There are a number of other exercises ranging from first aid equipment and how we work together."
National Olympic Security Co-ordinator, Assistant Commissioner Chris Allison, said: "Testing and exercising is vital to getting our safety and security operations for the Games right.
"We need to be confident that we have the right people in the right places, that we understand how others operate and that we are talking to each other at the right levels and in the right way."
As well as the response of emergency teams, the exercise is also designed to test co-ordination and communication between key strategic decision-makers who do not normally work together.
These include the police National Olympic Co-ordination Centre at Scotland Yard, the transport co-ordination centre and the London 2012 organisers' main operation centre.
The UK is currently on its third-highest alert level of "substantial", meaning a terrorist attack is a strong possibility.
This will almost certainly be raised to "severe", indicating an attack is highly likely, in the run-up to the Games.
The security drill was designed with this in mind but security minister James Brokenshire - who is taking part in the exercise along with Home Secretary Theresa May - stressed there is no evidence of a specific threat at this time.
"It's ensuring that we're testing, that we're really stretching our preparations as much as possible so when it comes to Games time, we're as ready as we can be," he said.
Some 13,500 troops and will be involved in the £1bn security operation for the London Olympics , which will run from July 27 to August 12, many of them reservists.
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