Emergency crews took part in a graphic mock-up of the aftermath of a building collapse into a Tube station to test their response to mass casualties.
Blood-covered "victims", fake dead bodies and actors with sliced limbs and open wounds were part of the biggest disaster training exercise ever seen in Europe.
The scenario, at a disused power plant near the Dartford river crossing in Kent, involved a mocked-up Waterloo Tube station being "crushed" by thousands of tonnes of rubble following a building collapse.
Following year-long planning, blue-light teams and other professionals were tested on their ability to deal with more than 1,000 casualties as seven Tube carriages were entombed.
Firefighters, police officers and ambulance staff were involved in the operation alongside London local authorities, Transport for London and the capital's air ambulance.
London Fire commissioner Ron Dobson said: "Exercises of this scale are important to ensure that we are always ready to respond no matter what happens.
"You can't get this sort of experience from a text book. We need to play it like it's real and ensure that should the worst happen, our response is effective and well-co-ordinated."
The teams' response would be "rigorously observed" by independent evaluators so lessons can be learned from the exercise, he added.
Exercise Unified Response, funded by the European Commission Exercise Program and co-ordinated by the London Fire Brigade, also involved utility companies and search and rescue teams.
Disaster victim identification (DVI) teams from all UK police regions were working alongside other forensic specialists.
In total, more than 250 personnel were working at the scene and in a specially-constructed temporary mortuary during the four-day exercise.
Chief Constable Debbie Simpson, of the National Police Chiefs' Council, said: "Victim identification is never a pleasant subject to discuss but it is unfortunately a reality.
"When disaster strikes, families need to be confident that the authorities are doing everything they can to identify their loved ones in a dignified and respectful way, whilst supporting any criminal investigation.
"Importantly this process cannot be hurried. As frustrating as this can sometimes be, especially in a world of fast paced mainstream and social media, we have to be meticulous in our approach to ensure we achieve reliable scientific identification.
"It's not often we get to test working practices on such a scale and it's really positive to see so many of our European colleagues involved. Effective evaluation and debriefing will help highlight good practice and any areas for development."