The training operation – involving more than 250 volunteers – took place at the West Yorkshire Police Carr Gate Complex in Wakefield.
It forms part of national series of exercises aimed at ensuring cities across the UK are prepared.
West Yorkshire Police, West Yorkshire Fire and Rescue and Yorkshire Ambulance Service joined NHS staff, Yorkshire Water staff and other agencies for the event.
Temporary chief superintendent Pat Casserly, of West Yorkshire Police, said: “This type of exercise is vital to ensure that we are properly prepared for any type of major incident and that we work together effectively.
“I can reassure members of the public that this is routine and has no impact on our frontline services.”
The scenario involves the deliberate release of a chemical, and focussed on the process of decontaminating people affected by an incident and their subsequent care and support in a survivor reception centre.
Deputy chief fire officer Dave Walton, of West Yorkshire Fire and Rescue, said: “The range of major incidents which emergency responders are faced with is massive.
“Opportunities to test and rehearse the multi-agency response on this scale are limited, and we will seek to learn as much as we can from the exercise as well as assuring ourselves that our response is effective.”
Mike Shanahan, head of special operations at Yorkshire Ambulance Service NHS Trust, said: “Exercises like this are an excellent opportunity to test our major incident procedures and partnership working with emergency service colleagues.
“The staff participating in the exercise are additional to our frontline staff already on duty so there will be no detrimental impact on normal services.”
The exercise is not a planned response to any specific threat but is one of a number of other similar exercises that have taken place across the country.
It has been in preparation since June last year and is one of a number of routine exercises organised to test the response of the emergency services.
Under the Civil Contingencies Act, there is a legal obligation to prepare and practice for a major incident.