Over 200 members of the emergency services’ full-time and volunteer staff were involved in the exercise, which closed off part of the town centre from 4 pm.
The exercise revolved around a terrorist device which had been detonated inside Cyril Le Marquand House and had released an anthrax-type powder, injuring many staff within the building.
Decontamination tents and mobile command posts filled The Parade, which had been sealed off from Charing Cross up to the General Hospital.
Ian Dulake, the Island’s emergency planning officer who co-ordinated the incident, hailed the exercise a success.
He said: ‘The individual emergency services will still be undertaking their debriefing exercises, but the initial feedback we have is that many, many important lessons have been learned.’