Thousands of emergency workers and government staff began a two-day exercise on Wednesday to practise dealing with a major terrorism incident during the 2012 London Olympics.
Volunteers posing as dazed travellers emerged from a London underground station, some coughing, some covered in fake blood and some on stretchers as police and emergency workers swarmed outside.
The exercise, which resembled scenes following suicide bombings in London in July 2005, is the most high-profile test of the emergency services' readiness to deal with a major incident during the July 27-August 12 Games.
It will also check whether lessons have been learned from the 2005 attacks, when four suicide bombers killed 52 people by detonating explosives on three underground trains and a bus.
An inquest into those deaths made numerous recommendations last year, including action to ensure better communications after hearing that radios used by police and paramedics did not work below ground.
Taking place at the disused Aldwych tube station in central London, Wednesday's drill involved more than 2,500 staff from the emergency services, government and other agencies dealing with a bomb attack on a train deep underground.
Media and the public could see victims of the attack being brought to the surface to be either treated or questioned, while firefighters with breathing apparatus and specialist units such as hazardous response teams headed inside.
Behind the scenes, government ministers were taking part to make sure communication and coordination between the various agencies and with Olympic chiefs would function smoothly should there be an attack during the Games.
It is part of our programme to ensure that when we come to the Olympic Games we're ready, we can approach this with confidence and ensuring we are leaving nothing to chance, Security Minister James Brokenshire told Reuters on Monday.
Britain has long been considered a target for Islamist militants while there is an enduring threat from dissident Irish republican groups who maintain an armed campaign against British rule of Northern Ireland.
Brokenshire said Wednesday's exercise was not a response to any specific threat while senior police officers have repeatedly said there is nothing to indicate any groups are planning to target the Games.
(Reporting by Michael Holden; Editing by Clare Fallon)