One man "died" during the training exercise
A simulated terrorist attack has been staged in Birmingham to test the reactions of the emergency services and the army.
They were asked to deal with the scenario of a highly toxic chemical being was released in the International Convention Centre.
It is the first of three exercises taking place across Europe.
Reactions will be analysed to help identify practices and procedures to improve emergency responses.
The exercise centred around a scenario that 150 people attending a multi-faith meeting in the ICC had been exposed to toxic gas.
All of the emergency services and the army were involved and the venue was evacuated as part of the exercise.
Most participants knew they were taking part in an emergency services exercise but were not given exact details.
They were told one elderly man had died and others showed symptoms of poisoning.
Some of the people taking part were given roles to play such as a terrorist or as someone exhibiting symptoms of nerve gas agent infection.
Among the volunteers was Tracey Tibbart, from Birmingham, who said: "It was really realistic - when a man fainted we thought he was just being poorly, we didn't realise it was part of it."
People "exposed to toxic gas" assemble outside the ICC
This was the first of three exercises taking place across Europe over the next two years, as part of an EU-funded initiative to improve resilience to terrorist attacks and also so researchers can establish the best way to distribute information in an emergency.
Dr Brooke Rogers, from the Department of War Studies at King's College London, said the exercise was designed to assess people's behaviour when they are placed in a dangerous situation.
"This could be something like the London bombing, 9-11, and the sarin [gas attack] in Japan," she said.
"In many ways it is the general public who are the 'first responders', and generally people stop to help each other.
"This isn't necessarily a good thing because it means contamination can spread.
In the sarin attack, she said, members of the public drove victims to hospital and in doing so took dangerous toxins with them.
Volunteer Joan Dudley, from Dudley, said people took it seriously.
"At first we were all a bit giggly," she said. "But when it started properly we all got swept up in it."
Ch Supt Chris McKeogh said: "This exercise provides a great opportunity for all of those involved to look at procedures in place for an emergency such as this and put them into action.
"This way, we can evaluate more effectively where we need to further develop our plans and procedures."