Emergency services and other agencies are carrying out a live exercise at locations in Brighton, Eastbourne and Mid Sussex today.
This is a large-scale exercise testing real-time response to a number of major incidents.
The exercise is taking place in public, albeit in designated areas, to make it as realistic as possible.
The activity will be centred around Brighton, Eastbourne and Mid Sussex.
Residents and businesses nearest the activity have been made aware although they should not be affected.
Exercise director Steve Voice, Sussex Police, said: "Local people may notice considerable emergency service activity in these areas today.
“They can rest assured that far from having anything to worry about, this is actually a prudent test of our major incident plans and how we work with partners.
"It is extremely beneficial to the police and other agencies to test contingency plans, as directed by the Home Secretary, in a practical way and to continuously improve our service.
"The scenarios we are testing are exceptional and are of a magnitude that allows us to test all the likely resources available.
"The exercise will be conducted in designated areas and disruption, other than maybe short delays to traffic caused by emergency vehicles responding to the exercise, will be kept to a minimum.
“The public will not be affected and can expect business as usual with emergency services responding to routine and emergency calls."
Nearly 20 partners - colleagues from fire, ambulance and police as well as some local authorities, health services, Highways Agency and others are taking part in the exercise.
Following the test in responding to the major incidents the exercise will continue tomorrow in Brighton with police working with the Coroner in testing victim identity and recovery.
Senior contingency planning and resilience manager at South East Coast Ambulance Service, Steve Carpenter said: "Exercises such as this are vital to test the already robust plans we have in place in the event of a major incident.
“We're looking forward to working with our fellow emergency services as well as NHS and voluntary service colleagues to rehearse our plans in a realistic environment.
“The exercise will further strengthen our strong relationships and enhance our preparation to assist local communities in the event of a serious incident."
Group manager Glenn Jones from East Sussex Fire and Rescue Service said: "We work closely with other emergency services on a regular basis but - thankfully - incidents are rarely on a large scale.
“It is therefore vital we get the opportunity to experience different situations and test out how we can co-ordinate our response alongside other emergency services.”