Emergency services attended a fake explosion.
Police, firefighters and paramedics all took part in the live training exercise at Preston Baracks in Lewes Road, Brighton.
The fake explosion took place just after 10am.
The exercise - timed to coincide with other fake incidents in Eastbourne and Mid Sussex - was intended to test the emergency services' responses major incidents.
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.